Bug 1234897 - Pin the version of nodejs used on Heroku

https://devcenter.heroku.com/articles/nodejs-support#specifying-a-node-js-version

package.json

@@ -6,6 +6,9 @@
     "url": "https://github.com/mozilla/treeherder.git"
   },
   "license": "MPL-2.0",
+  "engines": {
+    "node": "5.3.0"
+  },
   "dependencies": {
     "grunt": "0.4.5",
     "grunt-angular-templates": "0.5.7",