🐛 Fix undefined local variable iiif_print

There was a change in this version of IIIF Print that needed an update
to routes.rb.

Ref:
  - #614
  - notch8/iiif_print#302

config/routes.rb

@@ -10,6 +10,7 @@
   concern :oai_provider, BlacklightOaiProvider::Routes.new
 
   mount Hyrax::IiifAv::Engine, at: '/'
+  mount IiifPrint::Engine, at: '/'
   mount Riiif::Engine => 'images', as: :riiif if Hyrax.config.iiif_image_server?
 
   authenticate :user, lambda { |u| u.is_superadmin || u.is_admin } do