It's not really necessary to use rabbit, but the existing tests
weren't really using the stream components at all because
zipkin spans were being collected by spring-cloud-sleuth-zipkin.
Zipkin 0.4.1 obviates some duplication of configuration. It is also the
first version that's published as a docker image.
By switching to the published image of zipkin-java, we can be more
consistent, as `spring-cloud-sleuth-zipkin-stream` is derived from that,
not the scala query service.
Recent versions of zipkin and brave operate with http instead of scribe.
This changes the implementation accordingly, as well adds a sampler to
the POST endpoint.
A notable impact is that we no longer require a collector process, as
the zipkin server's POST endpoint is a collector.
