Swap env of maruku and kramdown dependencies.

Maruku is now sunsetted, i.e. a dev dep, not a runtime dep.
Kramdown is now the default, i.e. a runtime dep, not a dev dep.
Closes #2285.
Merge branch 'master' of github.com:jekyll/jekyll

* 'master' of github.com:jekyll/jekyll:
  Change Gemnasium extension
  Change Fury extension
  Change Travis extension