Update migrations for Django 1.7

I moved the old migrations folder to south_migrations per http://south.readthedocs.org/en/latest/releasenotes/1.0.html#library-migration-path which *should* work for all South 1.0 users without trouble. Users of older versions of South would have to change the SOUTH_MIGRATION_MODULES setting for sitegate to point to the new south_migrations directory.