failing run of "mix compile" for Nerves

gistfile1.txt

$ mix compile
Nerves Precompile Start
Nerves Env current
Nerves Env loaded
Nerves Precompile End
===> Command bare not found
** (Mix) Could not compile dependency :cf, "/usr/local/bin/rebar3 bare compile --paths "/Volumes/Fido/Local/u/rdm/Dropbox/Elixir/hello_nerves/_build/rpi2/dev/lib/*/ebin"" command failed. You can recompile this dependency with "mix deps.compile cf", update it with "mix deps.update cf" or clean it with "mix deps.clean cf"

Per jschneck, tried:

$ cp ~/.mix/rebar3 /usr/local/bin/rebar3
overwrite /usr/local/bin/rebar3? (y/n [n]) y
cp: /usr/local/bin/rebar3: Permission denied
[17:58 Tue Jul 12] ~/Dropbox/Elixir/hello_nerves
$ so !!

$ mix compile
Nerves Precompile Start
Nerves Env current
Nerves Env loaded
Nerves Precompile End
===> Compiling cf
===> Package cf-0.2.1 not found. Fetching registry updates and trying again...
===> Updating package registry...
===> Writing registry to /Users/rdm/.cache/rebar3/hex/default/registry
===> Generating package index...
===> [cloudi_service_oauth1:1.5.1] Only existing version of cloudi_service_db_riak is 1.3.3
  which does not match constraint ~> 1.5.1. Using anyway, but it is not guaranteed to work.
===> Writing index to /Users/rdm/.cache/rebar3/hex/default/packages.idx
===> Compiling erlware_commons
===> Compiling bbmustache
===> Compiling getopt
===> Compiling providers
===> Rebar3 detected a lock file from a newer version. It will be loaded in compatibility mode,
  but important information may be missing or lost. It is recommended to upgrade Rebar3.
===> Rebar3 detected a lock file from a newer version. It will be loaded in compatibility mode,
  but important information may be missing or lost. It is recommended to upgrade Rebar3.
===> Rebar3 detected a lock file from a newer version. It will be loaded in compatibility mode, but important information may be missing or lost. It is recommended to upgrade Rebar3.
===> Compiling relx
==> exrm
Compiling 11 files (.ex)
Generated exrm app
==> nerves
Compiling 5 files (.ex)
Generated nerves app
==> hello_nerves
Compiling 1 file (.ex)
warning: unused import Supervisor.Spec
  lib/hello_nerves.ex:7

Generated hello_nerves app