this.state.lastIdealStyles[i] fails when children are removed from StaggeredMotion

Hi,
It might be my particular use case (cant share the code unfortunately), but i can say that the children are propagated via `React.Children.map`.
When child is removed and react-motion attempts to lookup a style value via `this.state.lastIdealStyles[i]`, it fails.
I've fixed this in my fork of react-motion and will gladly put in a pr, but my fix is just a bandaid as other cases could have the children being reordered.
It would seem like a better approach would be to store the previous styles and velocities in a key store by the components key.

##### edit

found https://github.com/chenglou/react-motion/issues/26

Start at the StaggeredMotion logic where this.state.lastIdealStyles[i] is read, and trace how children from React.Children.map are matched to previous styles and velocities. Reproduce removal and reordering cases, then compare the behavior with the linked issue #26; done means children can be removed or reordered without a style lookup failure.

Written by the indexing model from the issue text.