Upgrading Jekyll to 4.0

[MattReimer]

This is going to have to happen sooner or later. It's necessary for a few features we need in our Gem themes as well as stability and reliability. Also we don't want to get too far behind.

I need to do this carefully so we can roll this out incrementally with clear instructions. More to follow.

UPDATE

https://pages.github.com/versions/

still at 3.9. Check back again later.

[MattReimer]

GHPages is locked to jekyll 3 so this is impossible for now. I'm leaving this here though because we will need to use it eventually..