This triggers an uninitialized constant error for me:
```
➜  redirector git:(add-whitehall-rake) ber whitehall:slug_check
rake aborted!
uninitialized constant Redirector::Site::MASK
/var/govuk/redirector/lib/redirector/site.rb:80:in `check_all_slugs!'
/var/govuk/redirector/lib/tasks/whitehall/slug_check.rake:7:in `block (2 levels) in <top (required)>'
Tasks: TOP => whitehall:slug_check
(See full trace by running task with --trace)
```