add php 7.3 compatibility

Author: ffalkone

composer.json

@@ -14,7 +14,7 @@
         }
     ],
     "require": {
-        "php": "~7.0.0|~7.1.0|~7.2.0",
+        "php": "~7.0.0|~7.1.0|~7.2.0|~7.3.0",
         "ext-json": "*",
         "guzzlehttp/guzzle": "~6.3",
         "symfony/serializer": "~3.3|~4.0",