Only advertise OpenSSH 6.4

GCM, UMAC and ETM added in 6.2, but due to a memory corruption vulnerability
in 6.2 and 6.3 by an insecure GCM implementation
  http://www.openssh.com/txt/gcmrekey.adv
we only recommend OpenSSH 6.4+
  http://www.openssh.com/txt/release-6.4