Fixed tests so they pass in Rails 4 and updated gemspec.

In the attached commits, I updated the features that were failing against Rails 4 and ensured that the tests also passed against Rails 3.2 still. Unfortunately, I wasn't able to run the Rails 3.0 and 3.1 tests via appraisal yet because of some dependency issues, but I believe they should work as well.

In the gemspec, I updated the dependency on Rails to be '>= 3.0' and database_cleaner needs to be greater than 1.0.0.RC1 for Rails 4 support.

This should resolve #244.