Running on rails 4 it should be:

   $ rails g migration add_uuid_to_postgresql

   class AddUuidToPostgresql < ActiveRecord::Migration
     execute("CREATE EXTENSION uuid-ossp;")
   end

   $ rake db:migrate && rake db:test:prepare

Then create a table as shown in the article, and it should work.

For this to work, the PG role that the Rails app is using has to be a superuser, as AFAIK only superuser roles can execute CREATE EXTENSION.

Does anyone consider this an issue? I have been using non-superuser roles within my Rails apps, and using an outside superuser role to add extensions with an external tool (like pgAdmin).

Yes, that's exactly I didn't include it in the migration in the example.

There's a shortcut in the Active Record PostgreSQLAdapter:

  enable_extension 'uuid-ossp'