audit.rb: require https for download.savannah.gnu.org

[vszakats]

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and download-mirror.savannah.gnu.org

These URLs now do support HTTPS. The first one (download.savannah.gnu.org) is a redirector, that may redirect to an insecure URL, the second one (download-mirror.savannah.gnu.org) is a direct secure download.

[DomT4]

Apologies for the week's delay on this simple PR, I've kind of been drowning in Homebrew commitments lately. Merged in df4982e, thank you @vszakats 😃