Switch Sass compilation to Ruby

For now, Lib-sass (which node-sass uses to complile Sass code) doesn't yet support:

1. [@extend](http://sass-lang.com/documentation/file.SASS_REFERENCE.html#extend)
2. properly expanded formatting
3. line-numbers

The first is important to fully support the [govuk_frontend_toolkit](https://github.com/alphagov/govuk_frontend_toolkit), the others because we are compiling for readability, not compression.

When it does, [which could be soon](http://www.solitr.com/blog/2014/01/state-of-libsass/), we'll switch back.