Switch to LibZMQ3 (fix broken install on OS X).

LibZMQ3 won't build on OS X---or won't, at least, with the latest version of
libzmq, installed with homebrew (`brew install zmq`). Switching to LibZMQ3
fixes this, but it might break other things.

I was *trying* to install exobrain to work on adding TravisCI integration (issue #21), but got stuck installing the dependencies (you weren't kidding about having a lot of them!). This doesn't break anything on my system, but it might on yours, depending on your libzmq version. Feel free to ignore if it does!