suggest: use ZMQ_SNDZMQ for zmq >= 3

GitHub: fixes #63

See also: http://www.zeromq.org/docs:3-1-upgrade

Reported by Naoya Inada. Thanks!!!

    groonga_suggest_httpd.c: In function ‘serve_threads’:
    groonga_suggest_httpd.c:596:51: error: ‘ZMQ_HWM’ undeclared (first use in this function)
                   zmq_setsockopt(threads[i].zmq_sock, ZMQ_HWM, &hwm, sizeof(uint64_t));
                                                       ^