Rails31 compatibility

These changes get this gem up to being compatible with Rails 3.1. I'm really just taking the changes from @brennandunn (https://github.com/brennandunn/preference_fu/pull/3) which were never merged in and then getting the test suite passing. In the process I ended up needing to bump the jeweler gem and I chose to bump the Rails dependency to 3.1. Not sure this if this is the right way to go as there may be folks using this gem on 3.0 who could no longer use it with this change. Might be better to use a > 3.0 dependency. Based on the number of downloads at http://rubygems.org/gems/preference_fu this gem is still being used by a few folks ;-)

@sleverbor would appreciate any input or suggestions you have here.