CONTRIBUTING: Update supported PHPUnit versions
CONTRIBUTING: Update supported PHPUnit versions

As of PHPCS 3.2.3, PHPUnit 7.x is also supported.

paulschreiber/WordPress-Coding-Standards,WordPress-Coding-Standards/WordPress-Coding-Standards,paulschreiber/WordPress-Coding-Standards,jrfnl/WordPress-Coding-Standards,jrfnl/WordPress-Coding-Standards,jrfnl/WordPress-Coding-Standards,paulschreiber/WordPress-Coding-Standards,jrfnl/WordPress-Coding-Standards,WordPress-Coding-Standards/WordPress-Coding-Standards,paulschreiber/WordPress-Coding-Standards,WordPress-Coding-Standards/WordPress-Coding-Standards,WordPress-Coding-Standards/WordPress-Coding-Standards