Replace node-sassy with node-sass (eliminate Ruby requirement)

This PR uses [node-sass](https://npmjs.org/package/node-sass), which is a wrapper for the [libsass](https://github.com/hcatlin/libsass) C implementation. This gets rid of the Ruby requirement, at the cost of `.sass`-file processing. In that regard it's kind of an opinionated PR, so I understand if you're not interested.