Adds reference to phpunit.xsd to phpunit.xml.dist

You might also want to refer to a specific PHPUnit version in the `composer.json` file under the `require-dev` section. That way contributing developers know which version you are writing tests against (as PHPUnit 4.0 has been released but most people still use 3.7).

You could also have Travis run the required version of PHPUnit instead of the default it uses now.