fixes #8366 - add parameter type to unique index

No, we only want to run this migration & add_index command if the user has the old 2-column index.

I implemented this to handle two scenarios: either the user succeeded in running 20140805114754, or they haven't yet run it.  If they succeeded, then this will replace the index.  If they haven't yet, then I changed the first and this second migration will do nothing.

(The reason being that the error seen in the ticket will prevent the first migration from running in some cases, so we have to change it.)