Make dependency configuration easier for people using Stomp

[wilkinsona]

http://info.michael-simons.eu/2015/11/17/spring-4-2-x-spring-boot-and-the-stomp-broker-relay/

I'm not sure that the dependencies should go in the WebSocket starter, but a Stomp starter might be useful

[michael-simons]

Stomp starter would be fine as well, sure!

[snicoll]

Duplicate of #6359