RC2 Update - Adjust phpbb version requirements

`3.1.*@dev` == `>=3.1.*, <3.2@dev`.

And it's not less than 3.2 dev, @dev is the minimum stability (so whether phpbb must be at least a development/rc/alpha/beta/stable version),

The reason for this change is the minimum requirement of the extension is RC2 because of this change. ;)