use libsass instead of sass rubygem

@zendesk/linksf 

Using [libsass](http://libsass.org/) instead of the sass rubygem, since it's faster.

![screen shot 2014-12-26 at 1 40 45 am](https://cloud.githubusercontent.com/assets/279406/5556644/5393bb36-8ca0-11e4-9171-bd2c451745f9.png)

I also added grunt-cli to the npm deps, since I've been using `export PATH=./node_modules/.bin:$PATH` in my `.bash_profile` to avoid globally installing. Let me know if you hate that.