Bugfix: Pin twig version as long as Twig-Extensions received no update

This prevents warnings when run in dev mode

composer.json

@@ -35,7 +35,7 @@
         "symfony/http-foundation": "^3.3",
         "symfony/psr-http-message-bridge": "^1.0",
         "twig/extensions": "^1.5",
-        "twig/twig": "^2.5",
+        "twig/twig": "~2.6.0",
         "zendframework/zend-diactoros": "^1.7"
     },
     "require-dev": {