React-transition-group: Add React 16 Support

[emilyrohrbough]

Feature Request

react-transition-group provides support for react v^15.0.0. React 16 is expected to be released soon and can currently be tested with react 16.0.0-rc (more information can be found on https://github.com/facebook/react/issues/10294).

react-transition-group is a dependency in a package in the terra-core monorepo, which will need to be React 16 compatible. From my initial assessment of our repository, it seems focus-trap behaves as expected with react v16.0.0-rc, but confirmed testing is needed.

[taion]

already have a PR in https://github.com/reactjs/react-transition-group/pull/198

[lukeapage]

@taion please can this be backported to v1 ? We have to use v1 because of velocity-react..
https://github.com/twitter-fabric/velocity-react/issues/224