Adding the engines config to package.json (#329)

Author: hiranya911

package.json

@@ -5,6 +5,9 @@
   "author": "Firebase <[email protected]> (https://firebase.google.com/)",
   "license": "Apache-2.0",
   "homepage": "https://firebase.google.com/",
+  "engines": {
+    "node": ">=6.0.0"
+  },
   "scripts": {
     "build": "gulp build",
     "lint": "run-p lint:src lint:unit lint:integration",