Rails 3.1.3 support

This commit updates the code to work with ActiveRecord / Rails 3.1.3, to address [issue #20](https://github.com/tlconnor/activerecord-postgres-array/issues/20). See the commit message for full details.

I'm not sure how you'd want to handle this from a versioning perspective, because this is _not_ forward-compatible. Let me know if you'd like me to change it to support both cases, or if you've got a better idea of how to handle it.

Thanks for the helpful gem!