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 test suites

ezmq connect and bind calls in the ezmq_zmq2 test suite have changed,
add 60 second timeouts for all test cases.
update travis config

run travis with an extra rebar.config to pull in erl_zmq2 for interop
tests.
libzmq2 compatible decoding of the greeting

ZeroMQ RFC 13 says IDFlags should be 0x00 in the greeting, however the actual
libzmq2 implementation sends 0x7E, so only insist on the LSB beeing zero.
Merge pull request #4 from RoadRunnr/master

improved travis CI support