Add marionette capability for Selenium2Driver

src/Behat/MinkExtension/ServiceContainer/Driver/Selenium2Factory.php

@@ -98,6 +98,7 @@ class Selenium2Factory implements DriverFactory
                 ->scalarNode('platform')->end()
                 ->scalarNode('browserVersion')->end()
                 ->scalarNode('browser')->defaultValue('firefox')->end()
+                ->booleanNode('marionette')->defaultFalse()->end()
                 ->booleanNode('ignoreZoomSetting')->defaultFalse()->end()
                 ->scalarNode('name')->defaultValue('Behat feature suite')->end()
                 ->scalarNode('deviceOrientation')->end()