Update PHPUnit to v9.4 (#15)

Author: PabloKowalczyk

composer.json

@@ -29,7 +29,7 @@
         "symfony/http-kernel": "^3.4 || ^4.4 || ^5.1"
     },
     "require-dev": {
-        "phpunit/phpunit": "^8.5"
+        "phpunit/phpunit": "^9.4"
     },
     "autoload": {
         "psr-4": {