Modify varnish to proxy audio

Summary:
Per T2583, we'd like to proxy audio in case people
have whitelisted domains. This modifies the varnish config
to theoretically do that. I would need to then replace
the URLs with these URLs on either the client or server.
I don't know how to test this locally, thoughts?

Test Plan:
- Pull, restart servers, make sure kastatic.org still works,
  and then test the URL in the varnish to see if audio loads

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D9469