[Urgent] Bug: Build package configs from main branch (or master)

[VersusFacit]

We need to ensure the hubcap script is prepared to handle packages with main, then prioritize that if there's no master branch. Currently, master is hardcoded into the script. We need main conditional logic that uses main then master when looking for commit shas to change into package specs. Otherwise, newer versions won't be added to hub.getdbt.com; the script will just break for any repo that does not retain a master branch.

Good news though! Because package specs depend only on commit shas, nothing has been changed for previous versions of packages.

This will partially address issue #69 .