GEVER 2.7.6: Fixed opengever.ogds.models pinning.
In opengever.ogds.models 1.0.3 the user groups key changed from `group_users` to `groups`.
Opengever.core got updated accordingly, but the pinning vor opengever.ogds.models is missing.
Merge pull request #15 from 4teamwork/lk_fix_models_pinning_in_core

GEVER 2.7.6: Fixed opengever.ogds.models pinning.