upgrade zmq dependency from 2.7.x to 2.8.x; zmq 2.7.0 build is broken.

Project does not compile because the dependency `zmq 2.7.x` does not compile.  See [zmq issue 316](https://github.com/JustinTulloss/zeromq.node/issues/316), solution in [issue 317](https://github.com/JustinTulloss/zeromq.node/issues/317).  This fix appears in `zmq 2.8.0`.  I've blindly upgraded the obelisk-client dependency on zmq to 2.8.0 and the project compiles again.  I have no idea whether this upgrade breaks anything else.  This is my first ever pull request.