Upgrade Jekyll and simplify deployment

Mark, for your review/testing. I'll do more content/css tweaks after this switchover. I can do the rest of the steps once you say the word.

* [x] upgrade from Jekyll 0.11 to 1.3
* [x] replace categories plugin so we can use GitHub Pages directly
* [x] remove yegrb.github.com repo
* [x] remove drone.io build webhook `https://drone.io/hook?id=github.com%2Fyegrb%2Fyegrb-jekyll.github.com`
* [x] rename this repo to yegrb.github.io