Fix the config structure for chrome prefs

This is a map node, not a list node.

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

@@ -146,7 +146,11 @@ class Selenium2Factory implements DriverFactory
                         ->arrayNode('switches')->prototype('scalar')->end()->end()
                         ->scalarNode('binary')->end()
                         ->arrayNode('extensions')->prototype('scalar')->end()->end()
-                        ->arrayNode('prefs')->prototype('variable')->end()->end()
+                        ->arrayNode('prefs')
+                            ->normalizeKeys(false)
+                            ->useAttributeAsKey('name')
+                            ->prototype('variable')->end()
+                        ->end()
                     ->end()
                 ->end()
                 ->arrayNode('extra_capabilities')