Adds rails_12factor gem for Heroku deploys.

Due to Rails 4.0 depreciating plugins, Heroku requires that you use the rails_12factor gem to properly handling logging and static asset serving on their platform now.
https://devcenter.heroku.com/articles/rails4-getting-started#heroku-gems

The gem does nothing on a Rails versions under 4.0, so it's safe to include in your Gemfile within the production stanza even with 3.2.x.  For simplicity sake, I updated the instructions on the installfest to include rails_12factor without having them check which rails version they are using.

If there are any other instance of instructions on how to setup a Gemfile for Heroku in other docs, they should also be updated accordingly.