Document instructions for upgrading to Heroku Stack-24+

[dbeatty10]

We should document the instructions from #335 into the "Heroku production setup" section of the RELEASE instructions.

Namely, prior to Heroku Stack-24, git was available at app run time. But starting with Heroku Stack-24, git needs to be installed separately in order to be used at runtime. Since hubcap.py relies upon git being available at app run time, we needed to do some extra setup in order for it to work.

Here are the upgrade instructions from Heroku.