title: Insert or update on table "django_admin_log" violates foreign key constraint when saving new model in admin
body: <p>When saving a new instance of a model in the admin, I get the following IntegrityError in a traceback:</p>
  
  <pre><code>Traceback:
  
  File "/venv/lib/python2.7/site-packages/django/core/handlers/base.py" in get_response
    149.                     response = self.process_exception_by_middleware(e, request)
  
  File "/users/dan/project/venv/lib/python2.7/site-packages/django/core/handlers/base.py" in get_response
    147.                     response = wrapped_callback(request, *callback_args, **callback_kwargs)
  
  File "/users/dan/project/venv/lib/python2.7/site-packages/django/contrib/admin/options.py" in wrapper
    541.                 return self.admin_site.admin_view(view)(*args, **kwargs)
  
  File "/users/dan/project/venv/lib/python2.7/site-packages/django/utils/decorators.py" in _wrapped_view
    149.                     response = view_func(request, *args, **kwargs)
  
  File "/users/dan/project/venv/lib/python2.7/site-packages/django/views/decorators/cache.py" in _wrapped_view_func
    57.         response = view_func(request, *args, **kwargs)
  
  File "/users/dan/project/venv/lib/python2.7/site-packages/django/contrib/admin/sites.py" in inner
    244.             return view(request, *args, **kwargs)
  
  File "/users/dan/project/venv/lib/python2.7/site-packages/django/contrib/admin/options.py" in add_view
    1435.         return self.changeform_view(request, None, form_url, extra_context)
  
  File "/users/dan/project/venv/lib/python2.7/site-packages/django/utils/decorators.py" in _wrapper
    67.             return bound_func(*args, **kwargs)
  
  File "/users/dan/project/venv/lib/python2.7/site-packages/django/utils/decorators.py" in _wrapped_view
    149.                     response = view_func(request, *args, **kwargs)
  
  File "/users/dan/project/venv/lib/python2.7/site-packages/django/utils/decorators.py" in bound_func
    63.                 return func.__get__(self, type(self))(*args2, **kwargs2)
  
  File "/users/dan/project/venv/lib/python2.7/site-packages/django/utils/decorators.py" in inner
    184.                     return func(*args, **kwargs)
  
  File "/users/dan/project/venv/lib/python2.7/site-packages/django/db/transaction.py" in __exit__
    223.                         connection.commit()
  
  File "/users/dan/project/venv/lib/python2.7/site-packages/django/db/backends/base/base.py" in commit
    242.         self._commit()
  
  File "/users/dan/project/venv/lib/python2.7/site-packages/django/db/backends/base/base.py" in _commit
    211.                 return self.connection.commit()
  
  File "/users/dan/project/venv/lib/python2.7/site-packages/django/db/utils.py" in __exit__
    95.                 six.reraise(dj_exc_type, dj_exc_value, traceback)
  
  File "/users/dan/project/venv/lib/python2.7/site-packages/django/db/backends/base/base.py" in _commit
    211.                 return self.connection.commit()
  
  Exception Type: IntegrityError at /admin/app/invitation/add/
  Exception Value: insert or update on table "django_admin_log" violates foreign key constraint "django_admin_log_user_id_52fdd58701c5f563_fk_auth_user_id"
  DETAIL:  Key (user_id)=(3) is not present in table "auth_user".
  </code></pre>
  
  <p>The exact same code works perfectly on another machine.</p>