Fix an error on require("firebase-node.js")

    Error: Cannot find module 'firebase-node.js'

worker.js and generator.js do `require("firebase-node.js")`.
However, in README.md, the instruction `npm install firebase` imply that requiring "firebase" from "node_modules".

This seems to be trivial for those who well experienced with node.js, but not for beginner like me.