[TASK] Set TYPO3 version to 6.2.6-dev

Change-Id: Ib005698d0b38bdac0d4a3d174a304058e4a55dbe
Reviewed-on: http://review.typo3.org/32940
Reviewed-by: TYPO3 Release Team <typo3v4@typo3.org>
Tested-by: TYPO3 Release Team <typo3v4@typo3.org>