Update phpunit to ~9.0

Author: srh

composer.json

@@ -17,7 +17,7 @@
         "php": ">=5.3.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "~4.8",
+        "phpunit/phpunit": "~9.0",
         "jakub-onderka/php-parallel-lint": "dev-master",
         "squizlabs/php_codesniffer": "~2.5"
     },