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.

⚡

Create Procfile from `npm start`

If no Procfile exists and there is a scripts.start in package.json, add
the scripts.start script as the 'web' process type to a new Procfile.

only prune when node_modules directory is restored from cache. Fixes #75

Add support for custom .npmrc

Make bin/release comply with published Buildpack API