Update package.json to support React 17

[alejandroiglesias]

Description

This updates the peerDependencies entry in the package.json file to support React 17 while keeping support for React 16.

Motivation and Context

Checklist

[x] I have not included any built dist files (us maintainers do that prior to a new release)
[ ] I have added tests covering my changes
[ ] All new and existing tests pass
[ ] My changes required the documentation to be updated
  [ ] I have updated the documentation accordingly
  [ ] I have updated the TypeScript 1.8 type definitions accordingly
  [ ] I have updated the TypeScript 2.0+ type definitions accordingly

[alejandroiglesias]

Closing as duplicate PR of #764.