Support Rails4.2

Hi!
I want to use this product in a Rails 4.2 application, so I've just implemented.

For Rails 4.2 supports,
- `require 'action_controller/responder'` #2 
- no arguments for `Garage::OptionalResponseBodyResponder#api_behavior`
Is this still needed? Now we will drop Rails3.x support by this pull request. I think we will not create this branch.