Explicitely return if db.dry_run is set in migration 0004.

Our version of south will run a migration regardless of whether db.dry_run is
set on MySQL. This tries to ensure that the migration can run correctly without
transactions because the RDBMS will not roll back properly.

See: https://github.com/theatlantic/django-south/blob/master/south/migration/migrators.py#L106-L113