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
It's gets generated when running `bin/setup`
https://github.com/thoughtbot/suspenders/blob/master/templates/bin_setup.erb#L26