Use newest api from ffi-rzmq

[paneq]

Breaking API Change from 0.9.3 to 0.9.5

The 0mq library functions zmq_send() and zmq_recv()
were changed to zmq_sendmsg() and zmq_recvmsg(
with the 0mq 3.x API change. I have changed the bindin
to always use #sendmsg and #recvmsg. This breaks
existing code. This should be the last breaking change
heading into the 1.0 release of these bindings.

chuckremes/ffi-rzmq#56

[mostlyobvious]

Libraries should have lesser restrictions on gem versions. We can implement strategies for different ffi-rzmq APIs.

[perplexes]

Can we detect which version of a particular gem is loaded at runtime?

[mostlyobvious]

@perplexes most of gems follows this policy: ruby -rffi-rzmq/version -e 'puts ZMQ::VERSION'

[paneq]

We do not use sendmsg and recvmsg methods from the binding itself. We relay on recv_string and send_string which were not changed between versions.