When using Composer's autoloader, you don't need to use Twig's internal one.

lib/pico.php

@@ -82,7 +82,6 @@ class Pico {
 
 		// Load the theme
 		$this->run_hooks('before_twig_register');
-		Twig_Autoloader::register();
 		$loader = new Twig_Loader_Filesystem(THEMES_DIR . $settings['theme']);
 		$twig = new Twig_Environment($loader, $settings['twig_config']);
 		$twig->addExtension(new Twig_Extension_Debug());