Point to rails github repo to get more green specs for now

Spree::Core::ControllerHelpers::RespondWith touches Rails
ActionController::MimeResponds::Collector#response which happens to
require a variant parameter on 4.1.0.beta1. Some other changes happened
after that though and it no longer requires a variant which make the
spree monkey patch apparently work again.

See https://github.com/rails/rails/commit/a288cc1e011cf25508d1b56f393023bb40ab441e
for reference