Updated libzmq CURVE to track RFC 27

* The INITIATE command vouch box is Box[C',S](C->S') instead of Box[C'](C->S),
  as recommended by https://codesinchaos.wordpress.com/2012/09/09/curvecp-1/,
  to reduce the risk of client impersonation.

* Mirrors the change in libcurve and CurveZMQ specifications.
Merge pull request #683 from hintjens/master

Updated libzmq CURVE to track RFC 27