Use stdio option instead of customFds

The customFds option in spawn is deprecated.
http://nodejs.org/docs/latest/api/child_process.html#child_process_child_process_spawn_command_args_options

Although it's actually converted for now, I don't want to see the deprecated message.
https://github.com/joyent/node/blob/master/lib/child_process.js#L808

Could you consider to merge this?