Fix inactive client reaping

wss.clients is a Set rather than an Array after ws 2.x
websockets/ws#934

src/server/websocket.coffee

@@ -61,7 +61,7 @@ exports.attach = (server, createAgent, options) ->
   # event, the client would have already established a new connection by the time
   # the server terminates it from it's side.
   setInterval ->
-    for client in wss.clients
+    for client in Array.from(wss.clients)
       client.terminate() if !client.isAlive
       client.isAlive = false
   , CLIENT_TIMEOUT