In development, don't allow earlier versions of phpunit

Author: Steve Bagwell

composer.json

@@ -10,6 +10,6 @@
         "php": ">=5.4"
     },
     "require-dev": {
-        "phpunit/phpunit": "^6.1 || ^7.0 || ^8.0"
+        "phpunit/phpunit": "^8.0"
     }
 }