Rails 4.2 support

Adds dependency on `responders` gem
Adds strategy for Rails 4.2
Remove unused Rails 4.1 strategy (fallback to 4.0)
Tests pass in 3.2, 4.0, 4.1, ~~4.2~~

See: https://github.com/apotonick/roar-rails/issues/87 for discussion.

Thanks @bengreenberg for the help.