Trouble updating GoCardless gem

Gemfile

source 'http://rubygems.org'

gem 'rails', '3.1.8'

# Gems used only for assets
group :assets do
  gem 'sass-rails'
  gem 'coffee-rails'
  gem 'uglifier'
  gem 'bourbon' # a comprehensive library of sass mixins (see https://github.com/thoughtbot/bourbon)
  gem 'compass-rails'
  gem 'zurb-foundation', '3.1.1' # http://foundation.zurb.com/
  gem 'therubyracer', :platform => :ruby
end

# Payments
gem 'gocardless'

# Hosting
gem 'heroku'

# Authentication and authorisation
gem 'devise'
gem 'cancan'

gem 'sentient_user'

# Attachments
gem 'carrierwave'
gem 'mini_magick'
gem 'fog'

gem 'state_machine'
gem 'validates_timeliness'
gem 'ice_cube', :git => 'git://github.com/seejohnrun/ice_cube.git' # ice_cube gem implements a recurring schedule (e.g. for bookings)

# Easily handle multiple nested models in forms
gem 'nested_form', :git => "git://github.com/ryanb/nested_form.git" # Contains fix for f.fields_for error

# Unobtrusive jQuery support
gem 'jquery-rails', '1.0.19'

# Background tasks
gem 'delayed_job_active_record'

# Soft delete - records are given a "deleted_at" date & time instead of being destroyed
gem 'paranoia'

# Spreadsheet exports
gem 'acts_as_xlsx'

# App monitoring
gem 'airbrake'
gem 'logglier'

# Database
gem 'pg'
gem 'schema_plus' # automatic foreign-key generation

# Performance
gem 'dalli' # Memcached

# Turn form select tags into search fields
gem 'select2-rails'

group :development, :test do
  # Testing
  gem 'rspec-rails'
  gem 'rb-fsevent', :require => false
  gem 'guard', :require => false
  gem 'terminal-notifier-guard' # Use Mountain Lion's Notifier for guard notifications
  gem 'guard-rspec'
  gem 'guard-spork'
  gem 'ruby-debug19'
end

group :development do
  # Console formatting
  gem 'wirble'
  gem 'awesome_print'

  gem 'guard-livereload', :require => false
  gem 'yajl-ruby'  # for guard-livereload performance improvement

  # Mail interceptor
  gem 'letter_opener'

  # Performance monitoring
  gem 'rack-mini-profiler'

  # Debugging
  # gem 'rails-footnotes', '>= 3.7.5.rc4'
end

group :test do
  # Testing
  gem 'cucumber-rails', :require => false
  gem 'capybara'
  gem 'factory_girl_rails'
  gem 'launchy'
  gem 'database_cleaner'
  gem 'mocha'
  gem 'spork', '> 0.9.0.rc'
  gem 'email_spec'
  gem 'launchy'
  gem 'shoulda-matchers', '~> 1.0.0.beta'
  gem 'timecop'
  gem 'capybara-webkit' # capybara-webkit requires Qt libraries.  See https://github.com/thoughtbot/capybara-webkit for Qt installation instructions
end

gistfile1.txt

Bundler could not find compatible versions for gem "rack":
  In Gemfile:
    rails (= 3.1.8) ruby depends on
      rack (~> 1.3.6) ruby

    gocardless (= 1.3.2) ruby depends on
      rack (1.4.0)