message: Cleared out scenes when animation's over
  React Motion calls it per item, React Move calls it for all together.
  Prefer React Move, but means have to clear out possible multiple scenes
  in one go
diff: @@ -94,11 +94,19 @@ class NavigationMotion extends React.Component<any, any> {
  }
  return sharedElements;
  }
  - clearScene(url) {
  - this.setState(prevState => {
  - var scenes = {...prevState.scenes};
  + clearScenes() {
  + this.setState(({scenes: prevScenes}) => {
  + var scenes = {...prevScenes};
  + var urls = this.getScenes().reduce((urls, {url}) => {
  + urls[url] = true;
  + return urls;
  + }, {});
  + for(var url in prevScenes) {
  + if (!urls[url]) {
  delete scenes[url];
  delete this.sharedElements[url];
  + }
  + }
  return {scenes};
  });
  }
  @@ -124,7 +132,7 @@ class NavigationMotion extends React.Component<any, any> {
  getKey={sceneContext => sceneContext.url}
  enter={sceneContext => this.getStyle(unmountedStyle, sceneContext, true)}
  leave={sceneContext => this.getStyle(unmountedStyle, sceneContext)}
  - // didLeave={({data: sceneContext}) => {this.clearScene(sceneContext.url)}}
  + onRest={() => this.clearScenes()}
  update={sceneContext => this.getStyle(sceneContext.mount ? mountedStyle : crumbStyle, sceneContext)}>
  {tweenStyles => (
  <View style={style}>
  