Audit check for MACOS_VERSION

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/cmd/audit.rb

@@ -385,6 +385,10 @@ class FormulaAuditor
     if text =~ /def options/
       problem "Use new-style option definitions."
     end
+
+    if text =~ /MACOS_VERSION/
+      problem "Use MacOS.version instead of MACOS_VERSION"
+    end
   end
 
   def audit