Add missing migraiton for later versions of Django CMS

[tdsymonds]

For later versions of Django & Django CMS there's a missing migration. When you try to run ./manage.py migrate Django gives the below message:

Your models have changes that are not yet reflected in a migration, and so won't be applied.
Run 'manage.py makemigrations' to make new migrations, and then re-run 'manage.py migrate' to apply them.

This looks like it's related to this issue.

This pull request adds that missing migration.