Better handling of south migrations for newer versions of south.

Hi,

We have tests that require `south` migrations to be run. This runner currently disables them and our tests fail.

In `south` >= 0.7.1 there is a helper in `south.management.commands` called `patch_for_test_db_setup` that inspects the Django configuration for a setting and either disables migrations or handles them.

This patch changes the `_disable_south_management_command()` method so that it uses this method.