Go to town with upgrades

[thatandromeda]

We're a full major version behind on jekyll and bundler. sass (used by our old jekyll) is end-of-lifed.

I would like to Upgrade All The Things. In the process, I'd also like to specify a ruby version, for ease of future developers. (I'll pick the most recent one consistent with jekyll and its friends.)

Any reason I should not do this?

[thatandromeda]

Hearing no objections, I've gone all lazy consensus on this and opened the referenced PR.