Mongoid 4 compatibility

This resolves #26. In Mongoid 4.0.0 (currently master), `#set` accepts a hash rather than two arguments. This pull request:

* Adds an additional Gemfile, `Gemfile.mongoid-four`
* Includes `Gemfile.mongoid-four` in `.travis.yml` for continuous integration
* Adds a method `ToCallback#relation_set` which uses the new format if the major version of Mongoid is `4`

Let me know if there are any additional changes that need to be made. Great gem! :)