FIX PHPUnit latest not working with composer installed builds

When using composer, we must rely on the composer autoloader to
load in PHPUnit and not try do to so ourselves, as the old
PHPUnit\Autoload.php file doesnt understand how to find things
in vendor

(This is the same fix as https://github.com/silverstripe/sapphire/pull/1117 but for the 3.1 branch)