serializer version bump

Author: Martynas Sudintas

composer.json

@@ -14,7 +14,7 @@
         "php": "^7.0",
         "symfony/symfony": "~3.0",
         "ongr/elasticsearch-bundle": "~5.0",
-        "jms/serializer-bundle": "^1.2"
+        "jms/serializer-bundle": "^2.0"
     },
     "require-dev": {
         "monolog/monolog": "~1.0",