Support zeromq 2.x and 3.x

[grantr]

If zeromq is used as a transport, both 2.x and 3.x should be supported.

[jordansissel]

Thinking about it now, I'm not sure zeromq support for external communication would be useful. lumberjack only uses it (for now) as internal thread-to-thread messaging.

If I decide later to support zeromq for external message publishing, I'll make sure it works with zmq 2 and 3.