Make Travis happy

https://travis-ci.org/zeromq/zmqpp/builds/42397641

src/zmqpp/zap_request.cpp: In constructor ‘zmqpp::zap_request::zap_request(zmqpp::socket&, bool)’:
src/zmqpp/zap_request.cpp:43:22: error: ‘z85’ has not been declared
         client_key = z85::encode(msg.get(6)); 
                      ^
make: *** [build/max-g++-4.8/obj/zmqpp/zap_request.o] Error 1