BACKWARDS INCOMPATIBLE: Fix dynamic model name for hvad compatibility.

The models generated by django-hvad don't have an underscore in their
names. This change affects projects that use South migrations, and
have ``translations = TranslatedFields(..)`` in their code.

To upgrade your apps, we recommend to generate a south migration,
and leave the ``forwards`` and ``backwards`` methods empty.
To create the migration, use::

    ./manage.py schemamigration appname --empty "upgrade_to_django_parler10"

Make sure there are no other changes in your models.