Warn about unsupported Node.js versions (#575)

* 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

global-cli/package.json

@@ -7,6 +7,9 @@
   "description": "Create React apps with no build configuration.",
   "repository": "facebookincubator/create-react-app",
   "license": "BSD-3-Clause",
+  "engines": {
+    "node": ">=4"
+  },
   "bugs": {
     "url": "https://github.com/facebookincubator/create-react-app/issues"
   },