Make increase_version _ aware and return original string upon failures

The VersionTagger fails in the spec file contains a number followed
by and underscore. This patch fixes that issue.
increase_version also returned an emtpy vlaue when nothing matched,
which led to a broken spec file. It now returns the original,
unmodified value instead.
Merge pull request #64 from spkane/master

Fix for increase_version that makes it underscore aware, and a bit more error tolerant