XML report without output

Hi,

Thank You for a great tool!

I've installed PhpMetrics 2.3.2 with composer.

When I run this command:

`vendor/bin/phpmetrics src --report-xml=build/phpmetrics.xml`

I get this output:

I get no output XML file.

And I expected this to happen:

I expected to get XML file, which i be able to use in Jenkins to plot some charts.
