Update SockJS CDN URL as is being retired.

Motivation:
The SockJS CDN will retire on 1 Dec 2014 [1] and we need up update our
netty-codec-sockjs and our documentation to reflect this.

JIRA: https://issues.jboss.org/browse/AGSMPLPUSH-68

[1] https://github.com/sockjs/sockjs-client/issues/198

This can be tested using the following steps. 
1. ```mvn clean install```
2. ```cd server-netty```
3. ```mvn exec:java```
4. ```curl -i http://localhost:7777/simplepush/iframe.html```
The response from the above curl command should be something similar to:
```shell
HTTP/1.1 200 OK
Content-Type: text/html; charset=UTF-8
Cache-Control: max-age=31536000, public
Expires: Sat, 24 Oct 2015 09:08:17 CEST
ETag: "2b50593460aa254330f5fad271aba653"
Content-Length: 489
Access-Control-Allow-Origin: *
Access-Control-Allow-Credentials: true

<!DOCTYPE html>
<html>
<head>
  <meta http-equiv="X-UA-Compatible" content="IE=edge" />
  <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
  <script>
    document.domain = document.domain;
    _sockjs_onload = function(){SockJS.bootstrap_iframe();};
  </script>
  <script src="http://cdn.jsdelivr.net/sockjs/0.3.4/sockjs.min.js"></script>
</head>
<body>
  <h2>Don't panic!</h2>
  <p>This is a SockJS hidden iframe. It's used for cross domain magic.</p>
</body>
</html>
```
The ```src``` of the script should have been changed from ```http://cdn.sockjs.org/sockjs-0.3.4.min.js``` to ```http://cdn.jsdelivr.net/sockjs/0.3.4/sockjs.min.js```.

This PR is targeted at [master](https://github.com/aerogear/aerogear-simplepush-server/tree/master) and the [0.12.x](https://github.com/aerogear/aerogear-simplepush-server/tree/0.12.x) branches.