remove CentOS7 from compatible OSes

PHP shiped with CentOS 7 is version 5.4. But now WordPress 5.2 requires at least 5.6.20. So CentOS7 acceptance tests are failing.

changed milestone to %3.0.0

created merge request !72 (merged) to address this issue

mentioned in merge request !72 (merged)

closed via commit d98fbbe4

changed title from remove CentOS7 support to remove CentOS7 from compatible OSes

mentioned in commit d98fbbe4