Update viewer.css

Added -webkit-backface-visibility: hidden; to the .none_hide and the show_hide. this fixes the transitions issue from one item to another.
Example videos plus 1 image in same placeholder the video stays blank after the first round.

Author:    woneal1983 <woneal@utm.edu>

Incremented app engine version
Merge pull request #23 from alex-deaconu/feature/css-update

Update viewer.css