Upgrade to react-transition-group v2

Breaking change!

Roe has a peer dependency on react-transition-group and so any projects using Roe will also need to upgrade react-transition-group to version 2.

The new version has a different set of components, renders a different wrapper element (div, not span), and has slightly different transition class names (-exit, not leave, and -[type]-done as a final class).

See #342 for examples of upgrading.