⬆️ bump minimum node version [4.5.0, 6.9.0] (#8489)

no issue

- the new ember version requires 4.5.0
- i've also bumped node v6, because of the latest security vulnerability report (see https://nodejs.org/en/blog/vulnerability/october-2016-security-releases/)
  "All of these vulnerabilities are considered low-severity for Node.js users, however, users of Node.js v6.x should upgrade at their earliest convenience."

package.json

@@ -23,7 +23,7 @@
     "init": "yarn global add knex-migrator ember-cli grunt-cli && yarn install && grunt symlink && grunt init || true"
   },
   "engines": {
-    "node": "^4.2.0 || ^6.5.0"
+    "node": "^4.5.0 || ^6.9.0"
   },
   "dependencies": {
     "amperize": "0.3.4",