Get rid of a compilation warning with newer (4.8) versions of cppzmq.

Fix #997 (closed).

Fix compilation warning: ‘int zmq::poll(zmq_pollitem_t*, size_t, long int)’ is deprecated: from 4.8.0, use poll taking std::chrono::duration instead of long

Get rid of the -1 parameter in the zmq::poll call. It's the default value, so the behavior is the same. Fix the deprecation warning coming with version 4.8.0 of cppzmq.