Bump ssh-keys dependency to resolve private key parsing bugs

[coelho]

russh uses a forked "ssh-key" crate available here: https://github.com/Eugeny/RustCrypto-SSH

The latest version of "ssh-key" upstream solves a bug parsing ECDSA/P-521 keys: RustCrypto/SSH#351 - There may be other fixes as well.

This dependency should be bumped.

[EpicEric]

The aforementioned PR has only been merged into the 0.7.0 channel, which is still unstable (currently at version 0.7.0-rc.0).