Fix Django DateTimeField warning on migrations

When running unit tests against my django apps, I am tired of seeing the warning from actstream's internal migrations that say: `DateTimeField received a naive datetime (2013-05-06 18:08:44.244158) while time zone support is active.`. This change removes the warning by using a workaround needed for South v0.7.6.
Merge branch 'master' of git://github.com/robbyt/django-activity-stream