Properly load hubot-scripts with NPM Hubot install

This builds on a prior __dirname patch from @keithduncan

https://github.com/github/hubot/pull/334
That pull should be made first

When Hubot is installed from NPM, the relative path of
../hubot-scripts.json is no longer valid

An npm install looks like

myhubot
  node_modules
    hubot
      bin
        hubot
    hubot-scripts
      src
        scripts
  hubot-scripts.json

Thefore the original paths specified here are not correct.  We don't
run on Heroku, so the original path resolutions are left intact, with
fallbacks to the NPM style install if things don't exist.

Opted for sync Fs API here since this is an initial load and won't
hurt perf.