[TASK] travis-ci on PHP7 only

TYPO3 CMS 8 will rely on PHP version 7. Previous PHP version
tests can be removed from travis-ci.

Change-Id: I976b33d8a50c6ab7a97a45e6828229df531a82cd
Resolves: #73509
Releases: master
Reviewed-on: https://review.typo3.org/46732


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>