* 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