Add some guardrail against tagging without updating Package-Version

[vermiculus]

Related to #159.

The current approach of using git describe --tags cannot work reliably. I originally thought this was due to the state in which Travis checks out the repository. Travis checks out each project with a detached HEAD, which means, as far as I could tell, that we can't get the closest tag to this commit. While it works on my development branch, even creating a new branch doesn't resolve the issue.

I'm leaving this issue open in the hopes that someone smarter than me can get this working. In the meantime, I'll just have to be more meticulous about releasing versions.