The updater is not smart enough for updates in two branches

[Konzertheld]

We came across the problem that 0.9.2 and 0.10 both needed an update, but because of our sequential numbering a user upgrading from 0.9.2 to 0.10 would get the update twice because the number that was next for 0.9.2 was already used in 0.10. So the updater should use something smarter than sequential numbering.

Part of the original discussion is in #593