Move away from the Sass ruby gem

[purrgrammer]

We are using the Ruby sass gem, which has been deprecated, and is causing issues when building the site in Travis. We should upgrade asap.

[purrgrammer]

I tried to move away from this in #80, however, the C bindings for libsass (called sassc, nice pun lol) are still not available in the official PPAs that the Travis Ubuntu version uses. The sassc package is available after 16.10 and Travis uses previous versions, so I'll come back to this in the near future.