Added @types/react-transition-group dependency
Added @types/react-transition-group dependency

It looks like this needs to be a dependency unless I want to make every
user using my library to also install @types/react-transition-group when
they install react-md. Since the type definitions don't exist even
though they are referenced in this package, they'll just be considered
required :/

mlaursen/react-md,mlaursen/react-md,mlaursen/react-md