I wonder if there's an easy way to make the validation script work with default branches as there was another pull request today that came in with two packages that do not use master.
The GitHub API to get repository details does return a default_branch property which is probably the quickest and easiest way to figure out the correct raw URL to the Package.swift.
as noted by @daveverwer:
referenced in daveverwer/SwiftPMLibrary #271