Set the first migration to depend on phpBB beta 4

I know, I know... it's bad practice to change an existing migration file. But I only do this because it's important!

I spent all day dealing with a major issue with the new test framework we switched to on another extension. While that issue does not appear to occur with this ext, the issue seems to be that if the first migration does not depend on a migration from the phpBB core, then it is possible that during the setup of the test environments, the boardrules migrations might run before the database is actually ready, resulting in missing table columns for example.

This change ensures the database is built up to the requirements of our migration files. It also has the added benefit of ensuring vanilla installs of this extension can only be done on 3.1-beta4, our current minimum requirement. This change should have no effect at all on anybody who might be upgrading from an older version of this ext so it should not pose any issue.