abort when $version (if set) and $tag do not align

version is (only) set by the scala-release-2.11.x-dist build flow.

This is also a backstop for https://github.com/sbt/sbt-git/issues/35.

Ignore empty version (job is running outside of the flow).
Merge pull request #95 from adriaanm/fullverprop

abort when $version (if set) and $tag do not align