[ticket/11253] Fix auth for signature module for updating from installations installed prior to #1025 being merged

That's a mouthful. It is better explained in the ticket, but basically the fix applied in #1025 only applies to 3.0.x updates, and 3.1-dev installs that occur after that patch was merged. This patch simply adds a migration to make sure that updating from a board running 3.1-dev from prior to #1025 (i.e. specifically, area51) gets the fix as well.

http://tracker.phpbb.com/browse/PHPBB3-11253