Travis: don't allow PHP 7.4 build to fail
Travis: don't allow PHP 7.4 build to fail

As [PHP 7.4 has been released](https://www.php.net/archive/2019.php#2019-11-28-1), the build against PHP 7.4 should no longer be allowed to fail.

WPTRT/WordPress-Coding-Standards,WPTRT/WordPress-Coding-Standards,WPTRT/WordPress-Coding-Standards,WPTRT/WordPress-Coding-Standards,WPTRT/WordPress-Coding-Standards