Teach pbr about post versioned dev versions.

Untagged versions should not be presented as final versions, which is
what was happening. The rules for dev versions are that they lead up
to the next release, so to emit a dev version we have to increment
the current version, then we can emit a dev version number.

implements: blueprint pbr-semver
sem-ver: feature
Change-Id: Icf2f1999613e0d26424798697de34811b9cfc4ab