Added gemfiles to test AR 3.1 and 3.2
Make Travis test only the versions that we actually support
Lock sqlite3 version so tests run with rbx
Silence more deprecations
Documentation improvements

- Add build status badge
- Mention that the gem also works for Rails 3.1
Move the AR dependency back to 3.1
Merge pull request #4 from andremedeiros/master

Testing with AR 3.1 and 3.2