Make it compatible with Rails 4.2

In Rails 4.2 has been changed a way verifying a request and validating an authenticity token. Compare
https://github.com/rails/rails/blob/master/actionpack/lib/action_controller/metal/request_forgery_protection.rb
https://github.com/rails/rails/blob/4-1-stable/actionpack/lib/action_controller/metal/request_forgery_protection.rb

My change brings a compatibility with Rails 4.2, 4.1 and 4.0. Please, release a new version after accepting this PR, if possible.