Update PHPUnit & PHPUnit polyfills.

Author: dd32

composer.json

@@ -29,9 +29,9 @@
 		"psr/log"                  : "~1.0"
 	},
 	"require-dev" : {
-		"phpunit/phpunit": "^7",
+		"wp-phpunit/wp-phpunit": "^6.8",
 		"spatie/phpunit-watcher": "^1.23",
-		"yoast/phpunit-polyfills": "^1.0"
+		"yoast/phpunit-polyfills": "^4.0"
 	},
 	"scripts"    : {
 		"test": "phpunit",