Update Twig too recent version (from 1.12 to 1.18)

composer.json

@@ -13,7 +13,7 @@
     ],
     "require": {
         "php": ">=5.2.4",
-        "twig/twig": "1.12.*",
+        "twig/twig": "1.18.*",
         "michelf/php-markdown": "1.3"
     },
     "autoload": {