attempt `+extension RANDR`

The `+extension RANDR` part is suggested at https://github.com/rogerwang/node-webkit/issues/769#issuecomment-35160608

The original Travis CI cookbook is here:

https://github.com/travis-ci/travis-cookbooks/blob/0d7955387bc317e4749e30f246044d67e05f445b/ci_environment/xserver/files/default/etc/init.d/xvfb.sh#L2

(The `x24` part is taken from the cookbook.)