RC2 Update - Adjust phpbb version requirements

We don't maintain BC between branches so we can't release something now and say it will be compatible with 3.2 as it might not be. This is how things are done. Plus I know of a 3.2 change that we know will break BC.

3.1.*@dev which everyone else is using is `less than 3.2` anyway so it's not a change in that respect, this is only a change in bumping the minimum with the same maximum as before.