Actual compatibility with Mongoid 4

I renamed `#mongoid4?` to `#mongoid_4?` to keep consistency with the naming of helper methods in the specs.

For some reason on [line 192 of spec/mongoid/alize/callbacks/to/one_from_one_spec.rb](https://github.com/gevans/mongoid_alize/blob/faf3747260a5866296653fe58acb45bbd6023031/spec/mongoid/alize/callbacks/to/one_from_one_spec.rb#L192), I have to reload `@person` to fix an undefined method error for `Head#substitute`. It likely has something to do with the way the relationship is built initially. It feels hackish.