Adding Support for PHPUnit v9.0

Author: ktamas77

composer.json

@@ -6,7 +6,7 @@
     "php": "^7.1",
     "webmozart/assert": "^1.2",
     "phpunit/php-code-coverage": "^4.0 || ^5.2 || ^6.0 || ^7.0",
-    "phpunit/phpunit": "^5.7 || ^6.0 || ^7.0 || ^8.0"
+    "phpunit/phpunit": "^5.7 || ^6.0 || ^7.0 || ^8.0 || ^9.0"
   },
   "autoload": {
     "psr-4": {