GEVER 2.7.6: Fixed opengever.ogds.models pinning.

In opengever.ogds.models 1.0.3 the user groups key changed from [group_users](https://github.com/4teamwork/opengever.ogds.models/blob/1.0.2/opengever/ogds/models/group.py#L28) to [groups](https://github.com/4teamwork/opengever.ogds.models/blob/1.0.3/opengever/ogds/models/group.py#L28).
Opengever.core got updated [accordingly](https://github.com/4teamwork/opengever.core/blob/3.2.4/opengever/ogds/base/contact_info.py#L159), but the pinning vor opengever.ogds.models is missing.

@phgross