Adapts this buildpack for the change to automatically run the package.json build script in the official Node.js buildpack.

Changes are:

• the Node buildpack executes npm run build automatically (this buildpack no longer runs the build)
• the Node buildpack installs devDependencies for auto build (and afterwards it prunes them), so a confusing NODE_ENV workaround has been removed
• the Node buildpack makes all config vars available during auto build, so compile-time configuration has access to all config vars.

See #156