title: Ruby 2.1.0 on existing Heroku application
body: <p>I got very weird problem with setting Ruby version to 2.1.0 on extisting Heroku app.
  When trying to deploy, got this error:</p>
  
  <pre><code> !     Invalid RUBY_VERSION specified: ruby-2.1.0
   !     Valid versions: ruby-2.0.0, ruby-1.9.3, ruby-1.9.2, ruby-1.8.7, ruby-1.9.3-jruby-1.7.0, ruby-1.8.7-jruby-1.7.0, ruby-1.9.3-jruby-1.7.1, ruby-1.8.7-jruby-1.7.1, ruby-1.9.3-rbx-2.0.0dev, ruby-1.8.7-rbx-2.0.0dev
  </code></pre>
  
  <p>On fresh Rails app deployed to Heroku, there is no problem with new Ruby version.</p>
  
  <p>My Gemfile:</p>
  
  <pre><code>source 'https://rubygems.org'
  
  ruby '2.1.0'
  
  gem 'rails',                        '~&gt; 3.2.14'
  gem 'pg'
  gem 'unicorn'
  gem 'pry'
  
  gem 'active_model_serializers'
  gem 'activeadmin'
  gem 'activerecord-postgres-hstore'
  gem 'acts-as-taggable-on',          git: 'git://github.com/mbleigh/acts-as-taggable-on.git'
  gem 'addressable',                  '~&gt; 2.3.2', require: 'addressable/uri'
  gem 'airbrake'
  gem 'alchemy-api-rb',               :require =&gt; ['alchemy_api', 'excon']
  gem 'bcrypt-ruby',                  '~&gt; 3.0.1'
  gem 'browser'
  gem 'cancan'
  gem 'ckeditor',                     '= 4.0.2'
  gem 'dalli'
  gem 'filepicker-rails',             git: 'https://github.com/limonka/filepicker-rails.git'
  gem 'geocoder'
  gem 'google_places'
  gem 'google_places_autocomplete'
  gem 'haml',                         '~&gt; 3.1.6'
  gem 'haml-rails',                   '~&gt; 0.3.4'
  gem 'heroku-api'
  gem 'kaminari'
  gem 'koala',                        '= 1.5.0'
  gem 'memcachier'
  gem 'metainspector',                '~&gt; 1.15.3'
  gem 'newrelic_rpm'
  gem 'jbuilder',                     '1.2.0'
  gem 'paper_trail'
  gem 'paperclip'
  gem 'pusher'
  gem 'rack-cache',                   require: 'rack/cache'
  gem 'rack-mini-profiler'
  gem 'recaptcha',                    require: 'recaptcha/rails'
  gem 'rest-client'
  # FIXME: Make sure that the bug with sidekiq-failures has been fixed: https://github.com/mhfs/sidekiq-failures/pull/49
  gem 'sidekiq',                      '= 2.15.2'
  gem 'sidekiq-failures'
  gem 'simple_form',                  '~&gt; 2.0.1'
  gem 'sinatra',                      '~&gt; 1.3.0', require: false
  gem 'slim',                         '~&gt; 1.3.8'
  gem 'sorcery'
  gem 'sunspot_rails',                '= 2.0.0'
  gem 'sunspot_solr',                 '= 2.0.0'
  gem 'state_machine'
  gem 'uglifier',                     '~&gt; 1.2.3'
  gem 'yajl-ruby'
  gem 'rack-cors',                    require: 'rack/cors'
  gem 'rubyzip',                      '~&gt; 0.9.9'
  gem 'paper_trail'
  
  # Gems used only for assets and not required
  # in production environments by default.
  group :assets do
    gem 'asset_sync'
    gem 'fog',                       '~&gt; 1.11.1'
    gem 'compass',                   '= 0.12.2'
    gem 'compass-rails',             '~&gt; 1.0.1'
    gem 'compass_twitter_bootstrap', git: 'https://github.com/vwall/compass-twitter-bootstrap.git', ref: '1a7b5ce5'
    gem 'coffee-rails',              '~&gt; 3.2.1'
    gem 'handlebars_assets',         '~&gt; 0.6.6'
    gem 'jquery-rails',              '~&gt; 2.1.4'
    gem 'sass-rails'
    gem 'coffee-script-source',      '~&gt; 1.6.2'
    gem 'turbo-sprockets-rails3'
  end
  
  group :test, :development do
    gem 'capybara'
    gem 'database_cleaner',           '~&gt; 0.7.2'
    gem 'factory_girl'
    gem 'faker'
    gem 'jasminerice',                git: 'git://github.com/bradphelan/jasminerice.git'
    gem 'minitest',                   '~&gt; 4.0'
    gem 'launchy'
    gem 'localtunnel'
    gem 'pusher-fake',                '= 0.9.0'
    gem 'ruby-prof'
    gem 'selenium-webdriver',         '~&gt; 2.34.0'
    gem 'webmock',                    '~&gt; 1.10.2', require: false
    gem 'mocha',                      require: false
  end
  
  group :development do
    gem 'bullet'
    gem 'foreman'
    gem 'growl'
    gem 'hirb'
    gem "letter_opener"
    gem 'mailcatcher'
    gem 'powder'
    gem 'thin'
    gem 'wirble'
    gem 'yaml_db'
  end
  </code></pre>
  
  <p>Any ideas?</p>