Revert "openssh: 6.9p1 -> 7.0p1"

This reverts commit a8eb2a6a. OpenSSH
7.0 is causing too many interoperability problems so soon before the
15.08 release.

For instance, it causes NixOps EC2 initial deployments to fail with
"REMOTE HOST IDENTIFICATION HAS CHANGED". This is because the client
knows the server's ssh-dss host key, but this key is no longer
accepted by default. Setting "HostKeyAlgorithms" to "+ssh-dss" does
not work because it causes ssh-dss to be ordered after
"ecdsa-sha2-nistp521", which the server also offers. (Normally, ssh
prioritizes host key algorithms for which the client has a known host
key, but not if you set HostKeyAlgorithms.)