Remove mysql and Rails 3.0 dependency

Project doesn't appear to be using mysql, so cut that gem. This is
working fine for me in a Rails 3.2.13 stack, so I'm setting the version
dependency at '> 3'.
Change library to use class's .fbuid column instead of .id

To decouple your users and Facebook's, I prefer adding fbuid as a column
on my User models. This commit updates the logic to account for that.

This commit also switches all the connection references to `REDIS`, to
work out of the box on Heroku, per:
  https://devcenter.heroku.com/articles/redistogo