Fix uninstall.rb brew remove --force "Directory not empty" error

Finder activity such as moving the position of an icon can create an unexpected
.DS_Store file in a Cellar directory. This causes `brew remove --force` to throw
an error that is reported as

Error: Directory not empty - /usr/local/Cellar/<formula>

This fix avoids that error, by calling rmtree rather than rmdir.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Library/Homebrew/cmd/uninstall.rb

@@ -20,7 +20,7 @@ module Homebrew extend self
               keg.rmtree
             end
           end
-          rack.rmdir
+          rack.rmtree
         end
       end
     end