formula_cellar_checks: Ruby 1.8 compatible check.

Library/Homebrew/formula_cellar_checks.rb

@@ -150,7 +150,7 @@ module FormulaCellarChecks
   private
 
   def relative_glob(dir, pattern)
-    return [] unless Dir.exist? dir
+    return [] unless Pathname.new(dir).directory?
     Dir.chdir(dir) { Dir[pattern] }
   end
 end