Switch typeplate to use a libsass-compatible version
Remove dependency on Bourbon for the time being

I only need it for the flexbox mixins, and I had to modify it a bit to avoid
some csslint warnings anyway.

Also added a better version of the prefixer() mixin, partially based on the
value-prefixer() mixin and to make it libsass-compatible.
Add support for Sass compilation via libsass-python

Removes a hard dependency on installing Ruby for GMusicProcurator (unless
you're developing it, in which case it's still required because of scss-lint).