drop the UNIX domain socket support

UNIX domain sockets are traditionally not used in Erlang. Supporting
them with a pur Erlang ZMQ implementation requires additional libraries
that purists might not want.
Also, the existing support is non-portable, breaking ezmq on non-Linux
platforms.
fix the ezmq connect and bind calls in the ezmq_zmq2 test suite
update travis config

run travis with an extra rebar.config to pull in erl_zmq2 for interop
tests.