Gemfile causes infinite loop with bundler

bundle env

## Environment

```
Bundler   1.14.6
Rubygems  2.6.11
Ruby      2.4.1p111 (2017-03-22 revision 58053) [x86_64-darwin16]
Git       2.11.0
Platform  x86_64-darwin-16
```

## Bundler settings

```
build.charlock_holmes
  Set for the current user (/Users/fukayatsu/.bundle/config): "--with-icu-dir=/usr/local/opt/icu4c"
```

## Gemfile

### Gemfile

```ruby
source 'https://rubygems.org'

gem 'sinatra-asset-pipeline'
gem 'sinatra-contrib'

# gem 'rack', '~> 1.0' # uncomment to fix infinite loop
```

### Gemfile.lock

```
<No /Users/fukayatsu/try/bundler-loop/Gemfile.lock found>
```

DEBUG_RESOLVER=1 bundle update -VV

...

                                            Creating possibility state for rack (~> 1.0) (3 remaining)
                                              Attempting to activate rack (1.1.0)
                                              Found existing spec (rack (2.0.3))
                                              Unsatisfied by existing spec (rack (2.0.3))
                                              Unwinding for conflict: rack (~> 1.0) to 22
                                            Creating possibility state for rack (~> 1.0) (2 remaining)
                                              Attempting to activate rack (1.0.1)
                                              Found existing spec (rack (2.0.3))
                                              Unsatisfied by existing spec (rack (2.0.3))
                                              Unwinding for conflict: rack (~> 1.0) to 22
                                            Creating possibility state for rack (~> 1.0) (1 remaining)
                                              Attempting to activate rack (1.0.0)
                                              Found existing spec (rack (2.0.3))
                                              Unsatisfied by existing spec (rack (2.0.3))
                                              Unwinding for conflict: rack (~> 1.0) to 21
                                          Creating possibility state for sprockets (~> 2.0) (2 remaining)
                                            Attempting to activate sprockets (2.0.1)
                                            Found existing spec (sprockets (3.7.1))
                                            Removing orphaned dependency rack (< 3, > 1) after swapping sprockets
                                            Activated sprockets at sprockets (2.0.1)
                                            Requiring nested dependencies (hike (~> 1.2), rack (~> 1.0), tilt (!= 1.3.0, ~> 1.1))
                                            Creating possibility state for rack (~> 1.0) (52 remaining)
                                              Attempting to activate rack (1.6.8)
                                              Found existing spec (rack (2.0.3))
                                              Unsatisfied by existing spec (rack (2.0.3))
                                              Unwinding for conflict: rack (~> 1.0) to 22
                                            Creating possibility state for rack (~> 1.0) (51 remaining)
                                              Attempting to activate rack (1.6.7)
                                              Found existing spec (rack (2.0.3))
                                              Unsatisfied by existing spec (rack (2.0.3))
                                              Unwinding for conflict: rack (~> 1.0) to 22
                                            Creating possibility state for rack (~> 1.0) (50 remaining)
                                              Attempting to activate rack (1.6.6)
                                              Found existing spec (rack (2.0.3))
                                              Unsatisfied by existing spec (rack (2.0.3))
                                              Unwinding for conflict: rack (~> 1.0) to 22
                                            Creating possibility state for rack (~> 1.0) (49 remaining)
                                              Attempting to activate rack (1.6.5)
                                              Found existing spec (rack (2.0.3))
                                              Unsatisfied by existing spec (rack (2.0.3))
                                              Unwinding for conflict: rack (~> 1.0) to 22
                                            Creating possibility state for rack (~> 1.0) (48 remaining)
                                              Attempting to activate rack (1.6.4)
                                              Found existing spec (rack (2.0.3))
                                              Unsatisfied by existing spec (rack (2.0.3))
                                              Unwinding for conflict: rack (~> 1.0) to 22
                                            Creating possibility state for rack (~> 1.0) (47 remaining)
                                              Attempting to activate rack (1.6.3)
                                              Found existing spec (rack (2.0.3))
                                              Unsatisfied by existing spec (rack (2.0.3))
                                              Unwinding for conflict: rack (~> 1.0) to 22
                                            Creating possibility state for rack (~> 1.0) (46 remaining)
                                              Attempting to activate rack (1.6.2)
                                              Found existing spec (rack (2.0.3))
                                              Unsatisfied by existing spec (rack (2.0.3))
                                              Unwinding for conflict: rack (~> 1.0) to 22
                                            Creating possibility state for rack (~> 1.0) (45 remaining)
                                              Attempting to activate rack (1.6.1)
                                              Found existing spec (rack (2.0.3))
                                              Unsatisfied by existing spec (rack (2.0.3))
                                              Unwinding for conflict: rack (~> 1.0) to 22
                                            Creating possibility state for rack (~> 1.0) (44 remaining)
                                              Attempting to activate rack (1.6.0)
                                              Found existing spec (rack (2.0.3))
                                              Unsatisfied by existing spec (rack (2.0.3))
                                              Unwinding for conflict: rack (~> 1.0) to 22
...