update node/npm to secure versions

package.json

@@ -6,6 +6,10 @@
     "type": "git",
     "url": "https://github.com/mozilla/pontoon-to-json.git"
   },
+  "engines": {
+    "node": "^4.8.4",
+    "npm": "^2.15.11"
+  },
   "bin": {
     "pontoon-to-json": "index.js"
   },