18F/hub-pages-private submodule at pages/private

Pages from pages/private will not be generated for public version of the Hub.
I'm not a fan of explicit returns in Ruby, or doing too much in one line. How about:

```ruby
unless @public_mode
  @site.pages.delete_if do |p|
    p.relative_path.start_with? '/pages/private'
  end
end
```