Lint Sass before committing code

I'll be out for a week, that'll make sure you behave :)

To know more about how scss-lint works, I [wrote an article on the developer blog](http://www.theguardian.com/info/developer-blog/2014/may/13/improving-sass-code-quality-on-theguardiancom)

![mind-blown-russell-brand](https://cloud.githubusercontent.com/assets/85783/2972534/61c7f80c-db75-11e3-8669-a2622845d21d.gif)