cleanup: skip rm_DS_Store during a dry run

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

Library/Homebrew/cmd/cleanup.rb

@@ -17,8 +17,10 @@ module Homebrew extend self
       end
       clean_cache
       # seems like a good time to do some additional cleanup
-      Homebrew.prune unless ARGV.dry_run?
+      unless ARGV.dry_run?
+        Homebrew.prune
         rm_DS_Store
+      end
     else
       ARGV.formulae.each do |f|
         cleanup_formula f