Add khanacademy.kilnhg.com to knownhosts so ssh doesn't prompt.

Test Plan:
Ran the command locally and then ran
   git clone --mirror ssh://khanacademy@khanacademy.kilnhg.com/Website/Group/webapp
and it started running successfully.

Reviewers: benkomalo, chris

Reviewed By: chris

Differential Revision: http://phabricator.khanacademy.org/D2219