Check Sass style

* Use open source SCSS-Lint https://github.com/causes/scss-lint
* Based on https://github.com/thoughtbot/hound/pull/406
* Use SCSS config from Bourbon Neat
  https://github.com/thoughtbot/neat/blob/master/.scss-lint.yml
In order to use a recent version of `scss_lint` we had to use a newer version of `bourbon` which supported `sass` > 3.4. Without those changes the UI was off.