lib/application: Remove forgotten `loopback` ref

Use `registry.Model` instead of `loopback.Model`.

lib/application.js

@@ -141,7 +141,7 @@ app.model = function (Model, config) {
     configureModel(Model, config, this);
     isPublic = config.public !== false;
   } else {
-    assert(Model.prototype instanceof loopback.Model,
+    assert(Model.prototype instanceof registry.Model,
       'Model must be a descendant of loopback.Model');
   }