[TASK] Add PHP 5.6 as travis-ci environment

travis-ci already provides a PHP 5.6 beta build.
It currently executes unit, functional and linting successfully
and is added to the list of standard environments for now.

Change-Id: Ibde2364564afe18f602f75174b779527b55c25bc
Reviewed-on: https://review.typo3.org/29913
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn