What causes the “Twig_Environment not found” error?

PHP Code:

@trigger_error('Using Twig_Autoloader is deprecated since version 1.21. Use Composer instead.', E_USER_DEPRECATED); 


I have switched to Twig-1.2.0 and now I get a "Class 'Twig_Extension_Debug' not found" message.


Filename: path\to\app\application\libraries\Twig.php

Line Number: 131