Unnecessary files in the slug

Trying to reduce the slug size, I investigated what ends up as part of the 86MB `.heroku/node` folder (as reported by `du -h -d 1 .heroku` in a one-off dyno).
Among this size, we have 6.3MB contributed by `.heroku/node/lib/node_modules/npm/docs`. Including the npm documentation in the slug looks unnecessary to me, and that represents 7% of the (uncompressed) size contributed by the buildpack.

Note that the `man`, `scripts` (containing npm development tooling) and `changelogs` folders of the `npm` install could probably also be removed in the cleanup (but they are much smaller).