formula_auditor: support non-master default branch for downgrade audit
formula_auditor: support non-master default branch for downgrade audit

nandub/brew,MikeMcQuaid/brew,nandub/brew,MikeMcQuaid/brew,nandub/brew,nandub/brew,Homebrew/brew,MikeMcQuaid/brew,Homebrew/brew,MikeMcQuaid/brew,Homebrew/brew,Homebrew/brew