Bump ZIPKIN_VERSION to 1.8.1
Bump ZIPKIN_VERSION to 1.8.1

openzipkin/docker-zipkin,jfeltesse-mdsol/docker-zipkin,itszero/docker-zipkin,tedyoung/docker-zipkin,openzipkin/docker-zipkin,tedyoung/docker-zipkin,jfeltesse-mdsol/docker-zipkin,itszero/docker-zipkin