Updating to the latest required gems.

https://help.github.com/articles/using-jekyll-with-pages defines the
current gems that are required to build pages.

Note that the jekyll command the docs should be tested with has changed
as well:

$ jekyll --pygments --no-lsi --no-auto --safe --server
Fix post metadata for jekyll 0.12.0.

Jekyll v0.12.0 removed the ability to have a category with an array.
Instead you must use 'categories'.