Update WebSocket samples to move away from the SockJS CDN

[wilkinsona]

The following is logged to the console when running spring-boot-sample-websocket in a browser:

*** SockJS CDN is being retired on Dec 1st *** Please transition to a public CDN. See sockjs/sockjs-client#198 for more information. (sockjs-0.3.min.js, line 27)