go.talks/present: log websocket close message to console, not popup

On platforms that don't support websockets, this makes the whole
experience a lot less obnoxious.

R=golang-dev
CC=golang-dev
https://golang.org/cl/10484043
diff --git a/present/js/socket.js b/present/js/socket.js
index 3fc7836..6e29c05 100644
--- a/present/js/socket.js
+++ b/present/js/socket.js
@@ -8,7 +8,7 @@
   var websocket, outputs = {};
 
   function onClose() {
-    window.alert('websocket connection closed');
+    console.log('websocket connection closed');
   }
 
   function sendMessage(m) {