Use kramdown as the markdown engine

Maruku's the default Kramdown engine that :ship:s with Jekyll, but there are better ones out there that are much less error prone and can hopefully avoid headaches like the one @haleyvandyck [experienced recently](https://travis-ci.org/project-open-data/project-open-data.github.io/builds/10281363) on #118.

This pull request asks GitHub to use Kramdown to process Markdown instead. Kramdown's about 5x faster than MaruKu, and is 500x times less finicky. Kramdown's interpretation of markdown is almost identical to MaruKu's which is why it's a good choice to move to over say something like RedCarpet.

There were a few slight syntax changes, but as far as I know, everything should look exactly as it did before. Can anyone confirm that?

Anyone have any strong opinions? @parkr?