AdminActions causes TransactionTestCase to break testing

[PetrDlouhy]

The new develop branch causes tests of other application to fail. After the commit a128eb6 the use of TransactionTestCase, even as simple as:

class TestIsLibero(TransactionTestCase):
    def test_true(self):
        self.assertTrue(True)

causes following errors of other tests:

______________________________ test_permission_needed[mass_update] ______________________________
tests/test_permissions.py:28: in test_permission_needed
    with user_grant_permission(admin, ['demo.change_demomodel']):
tests/utils.py:96: in __enter__
    self.group = get_group(permissions=self.permissions or [])
tests/utils.py:78: in get_group
    model__iexact=model_name)
.tox/d20-py34/lib/python3.4/site-packages/django/db/models/manager.py:82: in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
.tox/d20-py34/lib/python3.4/site-packages/django/db/models/query.py:403: in get
    self.model._meta.object_name
E   django.contrib.contenttypes.models.DoesNotExist: ContentType matching query does not exist.

[domdinicola]

now it's not failing anymore #175