Enhancement: Reference phpunit.xsd as installed with composer

Author: localheinz

tests/phpunit.xml

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
-	xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/6.2/phpunit.xsd"
+	xsi:noNamespaceSchemaLocation="../vendor/phpunit/phpunit/phpunit.xsd"
 	bootstrap="bootstrap.php"
 	colors="true"
 	failOnRisky="true"