Engine updates etc.

I had these changes (+ more) in my fork since some time ago and it runs excellent. Because of that this module are very active I have remade them in fine steps and saved some opinionated changes for later. I recommend you (spree team) to setup use of Travis, Gemnasium and Coveralls that play very nice now. New updated Coveralls actually check coverage changes when someone submit a pull request. Some tweaks to the Gemfile are required to make Travis happy but I only include few because it might be a better way doing it.