Made the multilingual-content-enabled test runner work with older
Djangos. Added South to translation registration exclusions.

Note that a different approach was needed to automate translation
field creation / population with pre-1.7 / post-1.7 test database
setup.

Also note that this code relies on some proposed modeltranslation
changes: https://github.com/deschler/django-modeltranslation/pull/274.