6362: Change config precedence order.

6362: Change config precedence order.

Before: environments/*.rb, Rails defaults, application.yml,
application.default.yml.

After: application.yml, application.default.yml, environments/*.rb,
Rails defaults.

This makes it possible to use application*.yml to control configs that
have Rails defaults. (application.default.yml file has had such
configs for ages, like assets.digest, but they have never been
effective.)