Prevent a paramiko AttributeError in SSHClient

There's a bug in the paramiko library that causes an AttributeError when
paramiko is looking for a known_hosts attribute on SSHClient.

https://github.com/paramiko/paramiko/pull/176
https://github.com/paramiko/paramiko/issues/214
https://github.com/paramiko/paramiko/issues/220

Should this be fixed upstream in paramiko instead of locally?