Scss lint update config

As per @ironsidevsquincy's suggestion, we now enforce usage of single quotes around strings in Sass.

Also, the colour format is: 6 digits / lowercase
- Good: `#ffffff`
- Bad: `#FFF`

![puppy-wtf](https://cloud.githubusercontent.com/assets/85783/3039482/e0f437bc-e0de-11e3-9399-480f288bfbc7.gif)