php: drop 5.6 and 7.0

Both 5.6 and 7.0 is currently on life support, as in only recieving
security related patches.

This will only continue until:
 - 2018-12-31 for PHP 5.6
 - 2018-12-01 for PHP 7.0

Both these dates are in the middle of the 18.09 lifecycle. So it would
be nice to not have them in the 18.09 stable release. Dropping them
now would also result in less maintanance in updating them.

The death dates can be seen on both these links:
 - https://secure.php.net/supported-versions.php
 - https://en.wikipedia.org/wiki/PHP#Release_history