[TASK] Sync the PHPUnit configuration file version with the package (#423)

Author: oliverklee

phpunit.xml

@@ -1,6 +1,6 @@
 <phpunit
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
-    xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/4.8/phpunit.xsd">
+    xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/5.7/phpunit.xsd">
     <testsuites>
         <testsuite name="Project Test Suite">
             <directory>tests</directory>