Fix south migrations error

Importing unicode_literals causes a problem in South when importing from django.db.models.fields because then the name of the model class that is passed into __import__ is unicode, rather than a byte string. I'm not quite sure why this causes a problem importing from this module but not others. However, removing the unicode_literals import from these migrations fixes the issue, and it doesn't look like the migrations have any literals that need to be unicode.