[TASK] Raise version to 1.6.1-dev

Change-Id: I62446953c5c4556b78252630167d08a15983d62a
Reviewed-on: http://review.typo3.org/35350
Reviewed-by: Xavier Perseguers <xavier@typo3.org>
Tested-by: Xavier Perseguers <xavier@typo3.org>