[TASK] #5176 Update maximum PHP version to 5.6

Change-Id: I55462880c69e8774252897691ca891aca895d588
Reviewed-on: http://review.typo3.org/34661
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>