Convert RunSQL migration commands from list to strings

Convert RunSQL migration commands from list to strings

The ability to use a list was added in Django 1.8 - compare

https://docs.djangoproject.com/en/1.7/ref/migration-operations/#runsql
https://docs.djangoproject.com/en/1.8/ref/migration-operations/#runsql

We don't need a list as we run only 1 SQL command. So convert to
strings. This helps unbreak Django 1.7.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Stephen Finucane <stephenfinucane@hotmail.com>