New version format:
* for releases, `x.y.z` (follows tag without leading `v`)
* for nightly builds, `x.y.z+N+hash` (previous version, not the upcoming one)
This means that each nightly build `aptly` would report
correct version now.
Version is now complied into the aptly binary, system tests
automatically check for current version, no need to update them
anymore.
2943422d5d