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
Merge pull request #1119 from silverstripe-rebelalliance/fix/phpunit/master

FIX PHPUnit latest not working with composer installed builds