Use rubocop to check/correct the engine_cart stanza

add rubocop checks on engine_cart so it's stanza passes rubocop checks.

```
Gemfile:41:5: C: Bundler/OrderedGems: Gems should be sorted in an alphabetical order within their section of the Gemfile. Gem coffee-rails should appear before sass-rails.

    gem 'coffee-rails', '~> 4.1.0'

    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```