Show that we support runtime 5.3 (#40)

Author: Nyholm

composer.json

@@ -12,7 +12,7 @@
     "require": {
         "php": ">=7.2.5",
         "ext-json": "*",
-        "symfony/runtime": "5.x-dev"
+        "symfony/runtime": "^5.3"
     },
     "require-dev": {
         "symfony/phpunit-bridge": "^5.2"