Minversion value should be <= current version

[phmccarty]

The swupd_create_update binary has a --minversion option, and its value can be any arbitrary integer. However, when minversion > current, and minversion is fixed for successive builds, every build N in the range current <= N <= minversion will be a full minversion bump. Hitting such a situation would most likely be unintentional, because the update stream would be a series of full rebuilds instead of delta updates.

It would be helpful for swupd_create_update to enforce that minversion <= current, because minversion > current is wasteful and not useful.