Use Kramdown in favor of Maruku

* Maruku is no longer maintained
  * http://benhollis.net/blog/2013/10/20/maruku-is-obsolete/
* Jekyll has replaced Maruku with Kramdown as the default
  * https://github.com/jekyll/jekyll/pull/1988/files
Merge pull request #259 from OpenHack/doug-use-kramdown

Use Kramdown in favor of Maruku