Fixes autoload error

Fixes the error  "PHP Warning:  require_once(SebastianBergmann/FinderFacade/autoload.php): failed to open stream: No such file or directory" by delegating autoload to vendor autoload of composer.

This error occurs on all install methods, except PEAR:
- a dev checkout
- tag dist download
- composer require-dev 
- and the PHAR distribution.