Don't fight against ourselves for the default log level

If we want to always default to `:debug`, let's just do that.

At which point the production.rb entry can become an "uncomment to change" instead.
@matthewd This would change the default for existing applications too, whereas the older implementation would keep the same default for existing applications, c/d? While this is *probably* okay, we should at least mention it in the upgrade guides.