Update composer.json

Author: freekmurze

composer.json

@@ -23,7 +23,7 @@
         "php": "^7.2",
         "ext-dom": "*",
         "phpunit/phpunit": "^8.0",
-        "symfony/yaml": "^5.0"
+        "symfony/yaml": "^4.0|^5.0"
     },
     "autoload": {
         "psr-4": {