Behat YML file for SauceLabs (not working)

sauce.yml

sauce:
     filters:
         tags: "@multibrowsers"
     extensions:
         Behat\MinkExtension\Extension:
             javascript_session: 'selenium2'
             base_url: http://www.legovaer.be
             selenium2:
                 browser: firefox
                 wd_host: <username>:<apikey>@ondemand.saucelabs.com/wd/hub
                 capabilities: { "platform": "Windows 2012 RC2", "version": "28", "name": "Behat test"}