Add compatibility for phpunit 8.4

In 8.4 the InvalidArgumentHelper, an `@internal` class was renamed. We
now point to the correct class. This still carries a risk for the
future, but the v8 branch should be safe to rely on this as no further
releases will remove this class.

Co-authored-by: bmstanley <bmstanley87@gmail.com>

Author: rdohms

composer.json

@@ -3,7 +3,7 @@
     "description": "This package provides Array Subset and related asserts once depracated in PHPunit 8",
     "type": "library",
     "require": {
-        "phpunit/phpunit": "^8.0",
+        "phpunit/phpunit": "^8.4",
         "php": "^7.2"
     },
     "license": "MIT",