audit: don't error on old OS X versions

Library/Homebrew/dev-cmd/audit.rb

@@ -918,6 +918,7 @@ module Homebrew
       newest_committed_url = nil
 
       fv.rev_list("origin/master") do |rev|
+        begin
           fv.formula_at_revision(rev) do |f|
             stable = f.stable
             next if stable.blank?
@@ -932,6 +933,9 @@ module Homebrew
             newest_committed_revision ||= previous_revision
             newest_committed_url ||= stable.url
           end
+        rescue MacOSVersionError
+          break
+        end
 
         break if previous_version && current_version != previous_version
         break if previous_revision && current_revision != previous_revision