added support for symfony 4 serializer

composer.json

@@ -12,13 +12,13 @@
     ],
     "require": {
         "php": ">=5.6",
-        "symfony/serializer": "~2.7|~3.0",
+        "symfony/serializer": "~2.7|~3.0|~4.0",
         "paragonie/random_compat": "^1.0|^2.0"
     },
     "require-dev": {
         "phpunit/phpunit": "~5.6",
         "squizlabs/php_codesniffer": "~2.0",
-        "satooshi/php-coveralls": "~1.0",
+        "satooshi/php-coveralls": "~1.0|~2.0",
         "elasticsearch/elasticsearch": "~5.0"
     },
     "suggest": {