Show that we support runtime 5.3 (#40)

Author: Nyholm

composer.json

@@ -13,7 +13,7 @@
         "nyholm/psr7": "^1.4",
         "spiral/roadrunner": "^2.0",
         "symfony/psr-http-message-bridge": "^2.1",
-        "symfony/runtime": "5.x-dev"
+        "symfony/runtime": "^5.3"
     },
     "require-dev": {
         "symfony/phpunit-bridge": "^5.2"