Remove unsupported schema fields from migrations

Migrations containing Mezzanine's KeywordsField, CommentsField, or
RatingField do not work with the latest versions of Django and
South.

(cherry picked from commit d2f538032c1eb0db96b78e3238e05d67f97aaea0)
Remove KeywordsField from urlconf_include

Related to d2f538032c1

(cherry picked from commit bb91717a5eaddae08ae4361b9be6cbd852863be9)