Upgrade Jekyll and simplify deployment

Mark, for your review. I'll do more content/css tweaks after this switchover.

* [x] upgrade from Jekyll 0.11 to 1.3
* [x] replace categories plugin so we can use GitHub Pages directly
* [ ] remove yegrb.github.com repo
* [ ] remove drone.io build webhook
* [ ] rename this repo to yegrb.github.io