[TASK] Raise version to 1.6.0-dev

Change-Id: I99af0aa8523f0da110515ba30ca6633a8ecfc63a
Reviewed-on: http://review.typo3.org/34249
Reviewed-by: Xavier Perseguers <xavier@typo3.org>
Tested-by: Xavier Perseguers <xavier@typo3.org>