[FEATURE] Use the testdox printer for PHPUnit

Also update the PHPUnit schema URL to the latest version and drop
a redundant attribute from the PHPUnit configuration file.

Fixes #236

Configuration/PHPUnit/phpunit.xml

@@ -2,8 +2,8 @@
 
 <!-- https://phpunit.de/manual/current/en/appendixes.configuration.html -->
 <phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
-         xsi:noNamespaceSchemaLocation="http://schema.phpunit.de/6.2/phpunit.xsd"
-         backupGlobals="false"
+         xsi:noNamespaceSchemaLocation="http://schema.phpunit.de/6.5/phpunit.xsd"
+         printerClass="PHPUnit\Util\TestDox\CliTestDoxPrinter"
          colors="true"
          bootstrap="../../vendor/autoload.php"
 >