Change the readme to specify a ref for your for of the websocket gem that contains the 'from_rack' method

Been playing around with tubesock this afternoon, and it took me a while to figure out why it was only working with v0.0.1 . I had copy + pasted the gemfile lines from the readme, so 'from_rack' wasn't being found. Not sure if you wanted to keep it pointing to a ref or just to the head, but I went ahead and replaced the ref with a suitable one