Added compatibility with the Symfony 4 serializer

composer.json

@@ -32,7 +32,7 @@
         "symfony/filesystem": "^2.8|^3.0",
         "symfony/config": "^2.8|^3.0",
         "symfony/finder": "^2.8|^3.0",
-        "symfony/serializer": "^2.8|^3.0",
+        "symfony/serializer": "^2.8|^3.0|^4.0",
         "doctrine/common": "^2.5",
         "doctrine/doctrine-bundle": "^1.6",
         "doctrine/doctrine-cache-bundle": "^1.2",