Do not insert the logger middleware when the logger doesn't have a formatter

Rails 3.2 ActiveSupport::BufferedLogger doesn't have a formatter. Inserting this middleware prevents Trasher to work with that version of Rails out of the box. Since the gem claims to work in Rails 3, I though it would be a good idea to fix it here, rather than patching my application.

@jeremy would you mind to cut a new version of the gem when this is merged?

Thanks
yup :+1: