sudo the rmdir on pkg uninstall

fixes an issue we've hit where chmod 777 is insufficient to allow
Pathname#rmdir to work properly.

refs #1374

lib/cask/pkg.rb

@@ -21,7 +21,7 @@ def uninstall
         _with_full_permissions(dir) do
           _clean_broken_symlinks(dir)
           _clean_ds_store(dir)
-          dir.rmdir if dir.children.empty?
+          _rmdir(dir)
         end
       end
     end
@@ -55,6 +55,12 @@ def info
     )
   end
 
+  def _rmdir(path)
+    if path.children.empty?
+      @command.run!('rmdir', :args => [path], :sudo => true)
+    end
+  end
+
   def _with_full_permissions(path, &block)
     original_mode = (path.stat.mode % 01000).to_s(8)
     @command.run!('chmod', :args => ['777', path], :sudo => true)