One month ago Heroku [announced websocket support](Websockets in public beta on Heroku).
If you need a pub/sub solution, Pusher remains an unbeatable alternative: its free plan is good for small apps, and you can setup it in seconds.
But if you need a classic client-server solution (i.e. to talk to each client separately) there were no chances to develop it on Heroku.
Now it should be easy enough to achieve this. You can find the official guide for ruby here.
And if you are wondering about performances, read this.