Require symfony/yaml

Author: Nyholm

composer.json

@@ -13,7 +13,8 @@
         "php": "^5.5 || ^7.0",
         "symfony/dependency-injection": "^2.3 || ^3.0 || ^4.0",
         "symfony/framework-bundle": "^2.3 || ^3.0 || ^4.0",
-        "symfony/http-kernel": "^2.3 || ^3.0 || ^4.0"
+        "symfony/http-kernel": "^2.3 || ^3.0 || ^4.0",
+        "symfony/yaml": "^2.3 || ^3.0 || ^4.0"
     },
     "require-dev": {
         "phpunit/phpunit": "^4.8.36 || ^5.5 || ^6.2"