added a note about when to remove a workaround

lib/Twig/Parser.php

@@ -69,6 +69,7 @@ class Twig_Parser implements Twig_ParserInterface
     {
         // push all variables into the stack to keep the current state of the parser
         // using get_object_vars() instead of foreach would lead to https://bugs.php.net/71336
+        // This hack can be removed when min version if PHP 7.0
         $vars = array();
         foreach ($this as $k => $v) {
             $vars[$k] = $v;