selenium test fix: yt embed name change (fix #2305)

The youtube embed used to be id="video-player-flash" now it's
id="video-player", breaking our selenium test.  This fixes it.
Remove elementtree from requirements (#2305)

Only thing requiring this was our DNS registration doohickey, and even
then it only required this package explicitly when xml.etree wasn't
available.  Best I can tell that is just prior to Python 2.5, which we
don't want to support.  Ergo safe to remove.
selenium test, remove errant import (sleep) (#2305)
Merge pull request #2308 from Stanford-Online/sef-2305-selenium-retry

Get selenium tests working on Travis (again) (#2305)