Declare undisclosed dependency on Rails 3.2.x

Gon depends on ActionDispatch::Request#uuid which was introduced in Rails 3.2.0: https://github.com/rails/rails/commit/afde6fdd5ef3e6b0693a7e330777e85ef4cffddb

It would be nice to warn people using Rails 3.1 that it's incompatible since 
ActionPack in 3.1 doesn't provide `request.uuid`. :-)