Fix ZMQ warning

The logic in this warning message was incorrect, as it would warn if
there is no zmq_version_info attribute in pyzmq, but then knowingly say
in the comment that pyzmq 2.1.9 doesn't have this attribute.

This uses zmq.zmq_version_info() if present, and if not then falls back
to obtaining the version string from zmq.version_info() and building a
version info tuple out of the result.