prop-types should not be a peer dep

[Sam A. Horvath-Hunt]

prop-types is listed as a peer dependency, however:

1. It isn't actually required at runtime as far as I can tell.
2. Many users (e.g. TypeScript users) won't be using it, so even if I'm wrong on the first point it probably shouldn't be required.

[Carmelo Pullara]

This has been updated in version 2. prop-types is no more a peer dependency.