react-dom should probably be a peerDependency

[boucher]

Summary:

React is being declared as a peerDependency, but react-dom is not (but both are dev dependencies).

Steps to reproduce:

1. Visit https://tonicdev.com/npm/react-modal and try to run the example

Expected behavior:

The package should be required. It will fail because of the missing peer dependency. If you add a require("react-dom") before hand, it will work.