Also react on `transitionEnd` event

This simply makes AnimationUtils also listen to the `transitionEnd` event. This future proofs the module in addition to supporting the non-WebKit prefixed property.
Note: it's ok if a browser supports both events and thus causes `finish()` to run twice, since Promises are already safe with redundant resolve() calls.  We might want to add a comment on `finish()` indicating that it may run multiple times though, so if anyone changes its code later they stay aware of that caveat.