send_tick: set "first" field (#3271)

According to the docs, the tick event should return:
    {
     "first": false,
     "payload": "arbitrary string"
    }

src/ipc.c

@@ -1127,6 +1127,9 @@ IPC_HANDLER(send_tick) {
 
     y(map_open);
 
+    ystr("first");
+    y(bool, false);
+
     ystr("payload");
     yajl_gen_string(gen, (unsigned char *)message, message_size);