Fix failing video tests due to wrong IFRAME_API_RESPONSE 

This fixes https://openedx.atlassian.net/browse/BLD-1194. 
The issue is due to change in http://www.youtube.com/iframe_api response javascript. We were using the hardcoded response due to which video player is not initializing. Similar to https://github.com/edx/edx-platform/pull/4691 but IFRAME_API_RESPONSE is hardcoded. It seems to make handout tests work fine.

Reviewers @polesye  @muhammad-ammar