audit: don't error on old OS X versions

[jonchang]

Some formulae are old enough to start taking driving lessons. Don't fail audits when the git history reaches into pre-history causing errors like:

==> brew audit viennacl --online --git --skip-style
==> FAILED
Error: unknown or unsupported macOS version: :snow_leopard

https://github.com/Homebrew/homebrew-core/runs/1401166825?check_suite_focus=true

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew tests with your changes locally?
• Have you successfully run brew man locally and committed any changes?

---

[MikeMcQuaid]

@jonchang Could you revert this and add this exception to

brew/Library/Homebrew/formula_versions.rb

Lines 12 to 16 in 2675fa0

	IGNORED_EXCEPTIONS = [
	ArgumentError, NameError, SyntaxError, TypeError,
	FormulaSpecificationError, FormulaValidationError,
	ErrorDuringExecution, LoadError, MethodDeprecatedError
	].freeze

instead? Thanks!