fix: add PHPUnit 8.x to composer.json to restore PHP 7.2 test support.

Author: pocketarc

composer.json

@@ -27,6 +27,6 @@
 	},
 	"require-dev": {
 		"mikey179/vfsstream": "1.6.*",
-		"phpunit/phpunit": "4.* || 5.* || 9.*"
+		"phpunit/phpunit": "4.* || 5.* || 8.* || 9.*"
 	}
 }