* Warn about unsupported Node.js versions
Add the `engines` field to package.json so users of old Node.js versions
will at least get a warning when trying to install create-react-app or
react-scripts, e.g.:
npm WARN engine create-react-app@0.3.0: wanted: {"node":">=6"} (current: {"node":"4.2.3","npm":"2.14.7"})
* Remove duplicated field and extra whitespace
* Change the engine version back to 4
Test Plan:
```
npm pack
node global-cli/index.js testing --scripts-version /Users/vjeux/random/react-getting-started/create-react-app-scripts-0.0.1.tgz
```
make sure it shows
```
Creating the app testing at /Users/vjeux/random/react-getting-started/testing
```