Mongoid no longer provides old .associations method on model class

only on instance, so it should be replaced with .relations

This fixes "undefined method `associations' for MyModel:Class" with current Mongoid git.

Method was an alias for .relations and is now deleted:
https://github.com/mongoid/mongoid/commit/22d7b0b6f45e8ce7d93753111833fa2e550f6e52#diff-e1722c18bb0d38e3b4f1d742e2537a6eL147