Add dependency on responders gem

In Rails 4.2, responders were moved out of core and into a gem, so this
dependency is required. It should be backwards compatible for all Rails
3.2+ versions.

As part of this change, the Responder class, and the workaround in the
README for Rails 4.2, needed to be changed to call #body on the Response
object.
Merge branch 'responder-dep' of github.com:bengreenberg/roar-rails into responder-dep