Fix ActiveRecord::Migration error in generated app
Fix ActiveRecord::Migration error in generated app

Rails 5 isn't happy that we inherited a migration directly from the `ActiveRecord::Migration` superclass. 

Instead, the new Rails convention is to inherit from the versioned Migration class, `ActiveRecord::Migration[5.1]`
alphagov/govuk-rails-app-template,alphagov/govuk-rails-app-template