Fix #309 Firefox starts instead of Chrome

Author: attrib

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

@@ -98,7 +98,7 @@ protected function getCapabilitiesNode()
                 ->scalarNode('platform')->end()
                 ->scalarNode('browserVersion')->end()
                 ->scalarNode('browser')->defaultValue('firefox')->end()
-                ->booleanNode('marionette')->defaultFalse()->end()
+                ->booleanNode('marionette')->defaultNull()->end()
                 ->booleanNode('ignoreZoomSetting')->defaultFalse()->end()
                 ->scalarNode('name')->defaultValue('Behat feature suite')->end()
                 ->scalarNode('deviceOrientation')->end()