[TASK] Disallow Composer installation with PHP 7.x

TYPO3 6.2 is not compatible with PHP 7.x thus properly declare this
for Composer installations.

Resolves: #73480
Releases: 6.2
Change-Id: I857a07199109f63b51079094d035b6f1ab9efb52
Reviewed-on: https://review.typo3.org/46707


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Nicole Cordes <typo3@cordes.co>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Reviewed-by: Daniel Goerz <ervaude@gmail.com>
Tested-by: Daniel Goerz <ervaude@gmail.com>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>