Bump minimal Node version to 6 (facebook#1897)

* Bump minimal Node version to 4.7.0

* Bump minimal Node version to 4.7.0

Modified additional files that needed a bump to 4.7.0 minimum node version

* Bump minimal Node version to 4.7.0

Reverse changes to packages/create-react-app/index.js as this file needs to continue to work on Node 0.10+

* Bump minimal node version to 6

* Bump minimal node version to 6

* Bump minimal node version to 6

package.json

@@ -5,7 +5,7 @@
   "repository": "facebookincubator/create-react-app",
   "license": "BSD-3-Clause",
   "engines": {
-    "node": ">=4"
+    "node": ">=6"
   },
   "bugs": {
     "url": "https://github.com/facebookincubator/create-react-app/issues"