Testing with AR 3.1 and 3.2

This PR makes the build pass on Travis CI.

Also:
- Silences a I18n deprecation warning that pops up halfway through the tests
- Improves the README file by adding a build badge and mentioning that the gem supports Rails 3.1 and 3.2
- Locks sqlite3 on 1.3.8 so that tests run on Rubinius
- Moves the ActiveRecord dependency back to 3.1, seen as the tests won't pass on 3.0