Accounting for change in Mongoid's metadata API.

From Mongoid's changelog:

>> `Document#metadata` has been renamed to `Document#relation_metadata` to
>> avoid common conflicts. Relation proxies also have this renamed to the
>> same as well.

This patch takes the same approach used for the `inc` method.  [All tests pass in CI](https://travis-ci.org/pjkelly/mongoid_orderable) without any changes.