Use url instead object in image_tag

Before Rails 5.2, it was possible to pass an uploader object to
`image_tag` and Rails guessed the url. With Rails 5.2 this is not
possible anymore.

app/views/admin/lodgings/index.html.haml

@@ -16,9 +16,9 @@
           - else
             - if lodging.website_link.present?
               = link_to(lodging.website_link, class: 'thumbnail') do
-                = image_tag lodging.picture.thumb, class: 'img-responsive img-lodging'
+                = image_tag lodging.picture.thumb.url, class: 'img-responsive img-lodging'
             - else
-              = image_tag lodging.picture.thumb, class: 'img-responsive img-lodging'
+              = image_tag lodging.picture.thumb.url, class: 'img-responsive img-lodging'
           .caption
             %h3.text-center
               = lodging.name