Use the same ID to unadvertise a topic as was used to advertise it

This was brought up in https://github.com/RobotWebTools/rosbridge_suite/issues/83

Previously, rosbridge couldn't unadvertise any Topics originating from roslib.js because different IDs were used to advertise and unadvertise. This change is tested on Hydro - a `ROSLIB.Topic` will not show up in `rostopic list` after it has been unadvertised.

Also adds instructions on how to get up-to-date Node.js on older Ubuntu versions