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