Support rails 4 via pre candidate.

Hello.

I'm using this gem for may project using rails 4. Unfortunately, this gem does not bundle with rails 4 because is expecting railties ~> 3.1. 

So I changed that and changed its version to add an pre. So by default it will support rails 3. But if I wanted to use it inside a rails 4 app I can explicit tell to get the "pre" version at the rubygems.