symfony/yaml used and should be explicitly required

Author: andrew-demb

composer.json

@@ -13,7 +13,8 @@
     "symfony/console": "^4.3|^5.0",
     "symfony/dependency-injection": "^4.3|^5.0",
     "symfony/event-dispatcher": "^4.3|^5.0",
-    "symfony/http-kernel": "^4.3|^5.0"
+    "symfony/http-kernel": "^4.3|^5.0",
+    "symfony/yaml": "^4.3|^5.0"
   },
   "minimum-stability": "dev",
   "prefer-stable": true