Added queue_size option as required in Indigo to prevent warning

Fix for https://github.com/RethinkRobotics/baxter_examples/issues/32

I'm not 100% sure if it should be value ``10`` or ``None``, but ``None`` doesn't remove the warning message 

@dirk-thomas why does not suppress the error even though it was the default in groovy and earlier? Do you know why was this change made in Indigo anyway when it makes ROS a bit more complicated?

Thanks!