Updates to get Rails4 version of Fulcrum working on Heroku

There were a number of small but niggly issues with getting back up on Heroku. Sorry I hadn't picked them up before.

Here's the basics:

Heroku expects there to be a /bin directory 
Heroku wants you to set the version of ruby in the Gemfile
Heroku wants you to use rails_12factor gem

The i18n-js isn't very Rails4+Heroku friendly so the assets directory needed versioning. This one I'm not sure why.