[TASK] Update maximum PHP version to 5.6

Resolves: #63354

Change-Id: I76c11fabfc15cd59f1908acbc64a043dce1631ce
Reviewed-on: http://review.typo3.org/34660
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>