update requirements to PHP 7.2 and WP 5.1

As discussed in #253 we already have some pending changes that require more recent PHP and WordPress versions.

Current requirements (up to 1.8.x):

WordPress 4.7
PHP 5.2

Minimum without reverting stuff would be PHP 5.3 (#247) and WP 5.1 (#252).

Maintaining and testing old stuff is time consuming and only very few sites actually have such requirements. With this change, we jump forward to:

WordPress 5.1
PHP 7.2

WP 5.1 does not play nice with PHPUnit 7/8 and neither with Polyfills, so let's just use 5.2 as minimum version in CI tests and rely on the static compatibility checks instead of investing time and effort in testing one more version.

pluginkollektiv / statify

update requirements to PHP 7.2 and WP 5.1 #287

Quality Gate passed