Use tag_regex for past releases

To make a tag_regexp like ^v(\d+\.\d+)$ work the regex must be
executed on every release version.
(like ChangelogFromGit.pm:393 does)

The debian tools will refuse a changelog version with a 'v' prefix.
Merge pull request #4 from benningm/fix-use-tag-regexp

Use tag_regex for past releases