fix unnecessary verbose migration

Whenever runserver or unittests are running, the following (IMHO unnecessary) messages are written to stdout:

"""
Adding permission 'pybb | Category | Can add Category'
Adding permission 'pybb | Category | Can change Category'
Adding permission 'pybb | Category | Can delete Category'
Adding permission 'pybb | Forum | Can add Forum'
...
"""

The attached patch fixes this.