Nodes that have had their child removed already, but then get given a new one no longer bug-out.

In the case of having an animated node which is, after the leave-transition has been activated, then re-added in a render call causes React to 'break'.  This is especially noticeable if you spam to removal and re-addition of an item in a ReactTransitionGroup.

This fix simply stops the leave transition and restarts the enter transition.
Updated the animation-fix to account for transitionEnter being false.
Merge pull request #642 from Cartas/master

ReactTransitionGroup can handle components being re-added before 'leave' transition has completed.