Support ssh-ed25519 SSH keys

Summary: Fixes T7121.

Test Plan: Used ssh-keygen -t ed25519 on an Ubuntu 14 box to generate a key; verified this is the header on the corresponding public key.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7121

Differential Revision: https://secure.phabricator.com/D11930