cache node dependencies, use latest stable node

- caches dependencies similar to
https://github.com/heroku/heroku-buildpack-nodejs
- removes hard-coded node version in favor of retrieving
version from https://semver.io
- vendor jq for parsing semver response
- caches bower_components
- add node_modules .bin to path
Merge pull request #29 from aortbals/cache-deps

Cache npm_modules and bower_components