fix ImproperlyConfigured raised when django.db.backends.dummy used with ...

...django 1.6

Celery worker dies as soon as it's started because `celery.fixups.django.DjangoFixup` calls `django.db.close_connection` (pending deprecation in django 1.6), it's replacement `django.db.close_old_connections` conveniently catches `DatabaseError` so all is good if we use that. https://github.com/django/django/blob/1.6/django/db/__init__.py#L67

As a side note, conn.close raises `AttributeError` because a `django.db.backends.dummy` 'connection' doesn't have a close method.