Update to jekyll v1.2.1

Jekyll v1.2.1 includes a very important bugfix for includes with Liquid 2.5.2. Upon update of Liquid to >= 2.5.2, this will be necessary. Does not affect Liquid 2.5.1.

Bumps version of RedCarpet to `~> 2.3.0` on Jekyll's end as well.

Close if this patch release isn't important to maintain the functionality of the current sites or if upgrading to Liquid 2.5.2 is undesirable.