Upgrading is only supported from latest point release

As per Nick Lewis, we only test upgrades from the "latest point release"
of each series.

E.g. upgrading from PE 3.0.0 -> 3.1.0 is *not* tested. Only upgrading from
3.0.1 -> 3.1.0 was tested. This is our standard practice.

It may work, but we should not claim that it does if we're not
explicitly testing it.