Make NonFlushingTransactionTestCaseMixin compatible with Django 1.5

This change patches the flush management command in
_fixture_teardown() as well as _fixture_setup().

    "Prior to 1.5, TransactionTestCase flushed the database tables
    before each test. In Django 1.5, this is instead done after the
    test has been run."

    https://docs.djangoproject.com/en/1.5/topics/testing/overview/#django.test.TransactionTestCase