Require v3.4+ of the var-dumper component

Author: skalpa

composer.json

@@ -20,7 +20,7 @@
         "ext-xml": "*",
         "symfony/http-kernel": "~2.8|~3.0|~4.0",
         "symfony/twig-bridge": "~2.8|~3.0|~4.0",
-        "symfony/var-dumper": "~2.8|~3.0|~4.0"
+        "symfony/var-dumper": "~3.4|~4.0"
     },
     "require-dev": {
         "symfony/config": "~3.3|~4.0",