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
Yes, specifically:

```
In Gemfile:
    scss-lint (>= 0) ruby depends on
      sass (~> 3.4.1) ruby

    neat (>= 0) ruby depends on
      sass (3.2.19)
```