build on travis with libzmq 2.1.11 and 2.2.0

libzmq-2.2.0 has this fix:

 * Fixed issue 293, libzmq doesn't follow ZMTP/1.0 spec.

This causes a visible change in the ZMTP/1.0 compatible greeting
(IdFlags 0x7E vs. 0x00). Make sure we cover that in the tests.
add Erlang 17.3 to the travis tests
small whitespace and comment cleanup
add sockname function, identical to inet:sockname for 0MQ sockets
run ezmq test suite on dynamic ports (using the new sockname function)

Note: this requires at least gen_listener_tcp v0.3.0