Two cards are swiped at once

[olvy]

Hi!

Thanks for great library, it is a good point for application to start from.

I have found minor issue when two cards are swiped at once.

Reproduction route:

• make two fast swipes to the right or left
• observe top and card below flickers instantly

captured it on video:
https://drive.google.com/open?id=0BwKq53rPhSpRcGFjSVZMVVdQcTQ

I thought that I may swipe the card below as well, but after debuging the code it
was found out that the onAnimationEnd() callback was called twice for the same card.

Sorry, don't have a permission to create pull request, here is workaround for this issue:

known issue:
https://groups.google.com/forum/#!topic/android-developers/U8s4s_-8i50

cc @Diolor