Upgrade jekyll / github-pages gem

This upgrades the github-pages gem to the latest version and remove compass in favor of the native sass compiler from jekyll. I still want to use some more stuff from this new Jekyll version but that would be enough for a separate PR (if what I have in mind will actually work).

I'd say we need at least one :+1: from a dev and a designer that checks out this branch, navigates through the blog comparing to the current version to make sure that nothing it broken before merging this PR

@Helabs/team I've already noticed that the syntax highlighting colors have changed and bad things might happen along the upgrade process if we don't take enough care, so please pay attention to the small details when reviewing the PR :heart:

Closes GH-245