Add Symfony 5 support

composer.json

@@ -12,14 +12,13 @@
     ],
     "require": {
         "php": "^7.0",
-        "symfony/serializer": "^3.0|^4.0",
+        "symfony/serializer": "^3.0|^4.0|^5.0",
         "paragonie/random_compat": "*"
     },
     "require-dev": {
         "elasticsearch/elasticsearch": "^6.0",
         "phpunit/phpunit": "~6.0",
-        "squizlabs/php_codesniffer": "^3.0",
-        "php-coveralls/php-coveralls": "^2.1"
+        "squizlabs/php_codesniffer": "^3.0"
     },
     "suggest": {
       "elasticsearch/elasticsearch": "This library is for elasticsearch/elasticsearch client to enhance it with DSL functionality."