Fix ensure_maildir

Fixes Steven Schmeier's error as discussed in sup-devel:

```shell
      undefined local variable or method `check_enable_experimental' for MaildirSub (deleted):Redwood::MaildirRoot::MaildirSub
      /usr/local/Cellar/ruby193/1.9.3-p545/lib/ruby/gems/1.9.1/gems/sup-999/lib/sup/maildirroot.rb:153:in `ensure_maildir' 
```