Change RespondWith to MimeResponds to match rails 4.2.0beta4
Otherwise, you get:

gems/actionpack-4.2.0.beta4/lib/action_controller/metal/mime_responds.rb:10:in `respond_to': The controller-level `respond_to' feature has been extracted to the `responders` gem. Add it to your Gemfile to continue using this feature: (NoMethodError)
  gem 'responders', '~> 2.0'

Even if the gem is installed.