Let PHPUnit Polyfills library manage PHPUnit installation

Author: dlh01

composer.json

@@ -15,11 +15,16 @@
   },
   "require-dev": {
     "alleyinteractive/alley-coding-standards": "^0.3.0",
-    "phpunit/phpunit": "7.5.*",
     "yoast/phpunit-polyfills": "^1.0"
   },
   "scripts": {
     "phpcs": "vendor/bin/phpcs",
     "phpunit": "vendor/bin/phpunit"
+  },
+  "config": {
+    "allow-plugins": {
+      "dealerdirect/phpcodesniffer-composer-installer": true,
+      "composer/installers": true
+    }
   }
 }