Use HTTPS for khan academy URLs when embedding in Perseus

Summary:
Programs wont load on live right now due to HTTPS on an HTTP url.
This fixes that.
(Could also go protocol-less, I suppose.)

Test Plan:
- Put program ID 6563968606273536 in the box, see it load.
- Wait for it to deploy to prod, try that again.

Reviewers: jack, alpert

Reviewed By: alpert

Subscribers: alpert, #cs

Differential Revision: http://phabricator.khanacademy.org/D11163