Support for npm 5 package-lock.json (#10)

* Add npm 5’s `package-lock.json`

* Upgrade deployment to Node 6.11

* Fix react-ui build command to work with npm 5.

* Revise react-ui build to honor lock file for production dependencies

package.json

@@ -3,11 +3,11 @@
   "version": "1.0.0",
   "description": "How to use create-react-app with a custom Node API on Heroku",
   "engines": {
-    "node": "6.9.x"
+    "node": "6.11.x"
   },
   "scripts": {
     "start": "node server",
-    "heroku-postbuild": "cd react-ui/ && npm install --only=dev && npm install && npm run build"
+    "heroku-postbuild": "cd react-ui/ && npm install && npm install --only=dev --no-shrinkwrap && npm run build"
   },
   "cacheDirectories": [
     "node_modules",