Test both ActiveRecord 3 and 4, and Mongoid version 2, 3, and 4.

I'd like to update this gem to support ActiveRecord 4, so Shopify can move to Rails 4 eventually. Unfortunately, this is not so easy because of conflicting dependencies of different versions mongoid and activerecord. I have updated the Travis config to use multiple Gemfiles with different version combinations that are allowed.

- ActiveRecord 3 & Mongoid 2
- ActiveRecord 3 & Mongoid 3
- ActiveRecord 4 & Mongoid 4 (beta)
- I've also decided to drop support for Ruby 1.8 (and ree)

I am planning to release this as version 0.2.0 after all the tests succeed.