update npm next readme

chore(README): Fix npm@next buildpack url

Prune any past devDependencies, if present in cache

Fixbower status

unset GIT_DIR

If the --production flag is specified, this command will remove the packages specified in your devDependencies.

https://www.npmjs.org/doc/cli/npm-prune.html

npm reverted to a backwards compatible cert

broken link to CONTRIBUTING.md

not saying this line should be removed necessarily, but it is broken. Is there a good `CONTRIBUTING.md` file we could add in here? Maybe from another buildpack?

the specified node runtime should be prior in the path

npm cert bug

Namespaced caching and node runtime version tracking

update compile script to work with new buildpack-env-dir API

Cache everything one level deeper (inside $cache_dir/node) to avoid clobbering cached artifacts from other buildpacks.
Store node-version textfile in new .heroku directory, in the slug _and_ the cache.

Make config vars available to npm install

Set default NODE_ENV to 'production'. Can't sit on the fence forever with this implementation.

Blow away the cached node_modules directory instead of the entire cache.