Make `hatchet.json` optional

[edmorley]

Currently I have to commit a hatchet.json containing {} even though I don't use any remote repo fixtures, since otherwise Hatchet will error:
https://github.com/heroku/heroku-buildpack-python/blob/5dcc5c4691904270267cc3d33416d6392caec6ca/hatchet.json

This file should be optional.