Updates README about cassandra duration query being unsupported
Updates README about cassandra duration query being unsupported

cassandra3 should be used per #1283

See #1531

openzipkin/zipkin,openzipkin/zipkin,adriancole/zipkin,openzipkin/zipkin,adriancole/zipkin,adriancole/zipkin,adriancole/zipkin,twitter/zipkin,openzipkin/zipkin,twitter/zipkin,twitter/zipkin,openzipkin/zipkin,twitter/zipkin