Update Twig from 1.35.0 to 1.42.4 (PHP 5.5 is now required!)

This fixes some errors on PHP 7.4 and contains even more fixes
Also bumped PHP version to 5.5 as Twig requires it.

system/libs/Twig/Source.php

@@ -9,6 +9,8 @@
  * file that was distributed with this source code.
  */
 
+namespace Twig;
+
 /**
  * Holds information about a non-compiled Twig template.
  *
@@ -16,7 +18,7 @@
  *
  * @author Fabien Potencier <fabien@symfony.com>
  */
-class Twig_Source
+class Source
 {
     private $code;
     private $name;
@@ -50,4 +52,4 @@ class Twig_Source
     }
 }
 
-class_alias('Twig_Source', 'Twig\Source', false);
+class_alias('Twig\Source', 'Twig_Source');