Upgrade React and React Native at the same time

Summary:
**Motivation**
As NoelBroda pointed out in https://github.com/facebook/react-native/pull/11104 and https://github.com/facebook/react-native/pull/11123, NPM often produces an unmet peerDependency when upgrading React Native. It causes a failure when running "npm install" with NPM2.

During the git-upgrade, we have to take care of the `react` peerDep: this PR checks if the installed `react` package matches the `react` peerDep of the new `react-native` version. If so, R & RN are upgraded at the same time (in the same `npm install`).

**Test plan**
- Publish `react-native-git-upgrade` to Sinopia,
- Run `react-native-git-upgrade` inside a RN project with version < 0.37,
- Verify that no "unmet peer dependency" warning is displayed
- Open the `package.json` and verify that both R & RN have been updated
Closes https://github.com/facebook/react-native/pull/11226

Differential Revision: D4258007

Pulled By: mkonicek

fbshipit-source-id: cff466d4710807d97fc6161f47bb974097b75261