Adds rake file for running deploys to heroku

New and improved Heroku deploys, which will actually run the migrations!

Also adds rollback support via git tags (tags HEAD after each deploy)

### Usage
Deploy:
`bundle exec rake deploy`

Rollback:
`bundle exec rake heroku:rollback`


You may see a warning about the heroku gem being deprecated, that's because it is, but I had to add it to the gem file so that bundler would play nice with it.  We can fix this by changing some bundler setting somewhere, but this was easier.

Ping @joshpc