Use a separate (blank) array of ignored exceptions for Rake tasks

From #220:

> We've just had a cron task failing silently for over a week with an `ActiveRecord::RecordNotFound` error, because the rake exception handler calls `notify_or_ignore`, and `ActiveRecord::RecordNotFound` is ignored by default.

> I propose a `@ignore_rake` configuration option, which defaults to an empty array. This could be used by changing `lib/airbrake/rake_handler.rb:16` to `Airbrake.notify_or_ignore(..., :ignore => Airbrake.configuration.ignore_rake`.