Fixed an issue related to composer installs requiring `PHPUnit/Autoload.php`.

If a composer project required `PHPUnit/Autoload.php` outside of a `vendor/bin/composer` run, you would get errors similar to #848.

The PHPUnit_Mock_Objects project, for example, was untestable with a composer-based install as it included .phpt files which require `PHPUnit/Autoload.php`.

This change also makes the `PHPUnit/Autoload.php` more consistent with `composer/bin/phpunit`.