update composer.json

Author: fenric

composer.json

@@ -24,7 +24,7 @@
         "phpunit/phpunit": "7.5.1",
         "sunrise/http-message": "^1.0",
         "zendframework/zend-diactoros": "^2.2",
-        "symfony/validator": "^5.0"
+        "symfony/validator": "^4.0"
     },
     "autoload": {
         "psr-4": {