Use Rails 5 syntax to set referer in specs

The previous way was working fine with Rails 4, but now the referer was
returning nil and therefore raising an error in this spec.

spec/controllers/remote_translation_controller_spec.rb

@@ -9,7 +9,7 @@ describe RemoteTranslationsController do
       @remote_translations_params = [{ remote_translatable_id: debate.id.to_s,
                                        remote_translatable_type: debate.class.to_s,
                                        locale: :es }].to_json
-      allow(controller.request).to receive(:referer).and_return("any_path")
+      request.env["HTTP_REFERER"] = "any_path"
       Delayed::Worker.delay_jobs = true
     end