rails-footnotes breaks Rails 4 application on boot

It seems the implementation of ActiveSupport::LogSubscriber has changed and at the very least you need to call super now when subclassing it. If you dont do this, the following error appears on trying to start the app:

```
ActiveRecord::StatementInvalid in DashboardController#index
TypeError: nil is not a symbol: SHOW FULL FIELDS FROM `subdomains`
```

This pull request only fixes the breakage on rails 4, but there are some other things not working as they should because the DB and Queries notes are not showing up in the footer.

For more information, also see https://github.com/rails/rails/issues/9433