Fix broken VERSIONSTRING handling

This was broken in:

| % git show 886054a89d8dbd2e003931a906cd1b7a56519fdf
| commit 886054a89d8dbd2e003931a906cd1b7a56519fdf
| Author: Thomas Lange <lange@informatik.uni-koeln.de>
| Date:   Tue Apr 23 14:26:38 2013 +0200
|
|     use dpkg-parsechangelog
|

Closes: http://jenkins.grml.org/job/fai-binaries/61/testReport/?