Fix for missing argument notice

Fixed issue #327 [Missing argument 3 for wpseo_upgrader_process_complete](https://github.com/Yoast/wordpress-seo/issues/327) notice for WP 3.7+

Originally also contained unscheduling of yoast tracking cronjob, but I have moved that to a separate branch & pull request (#344) to make sure this fix can be committed now.