Cannot build demos on react v16

Hi.

I've tried to build these demos as the readme.md, however they produce many failures as below:

```
ERROR in ./demos/demo0-simple-transition/index.jsx
Module not found: Error: Cannot resolve module 'react/lib/ReactMount' in D:\Github\react-motion\demos\demo0-simple-transition
@ ./demos/demo0-simple-transition/index.jsx 1:321-352
```

To resolve this, I fixed react and react-dom version to v15.5.0. but it seems not to be the best way.
Could you please make this demos buildable on react v16?

Thanks.

Start with README.md and the demo entry point shown in demos/demo0-simple-transition/index.jsx, then reproduce the build using React v16. Check the reported ReactMount module-resolution failure and the other demo build failures; done means the demos build successfully on React v16 without requiring React v15.5.0.

Written by the indexing model from the issue text.