Update ffi-rzmq to ~> 2.0.1 to allow zeromq 4.x

zeromq 4 is the latest stable and ffi-rzmq 1.0.1 only supports up to the 3.x
series.
Merge pull request #25 from zimbatm/ffi-rzmq-update

Update ffi-rzmq to ~> 2.0.1 to allow zeromq 4.x