Upgrade to coffee-rails 4.1.0

Upgrades default Gemfile coffee-rails version to 4.1.0.

The preferred extension format is now just `.coffee` instead of `.js.coffee`.

See Also

* https://github.com/rails/coffee-rails/pull/61
* https://github.com/rails/coffee-rails/issues/62

cc @rafaelfranca @spastorino @guilleiguaran