Move default production database to URL sub key

[schneems]

By using the URL sub key in the database.yml by default we are exposing the ability to set other attributes such as pool or reap_frequency without need of modifying the URL to contain non-connection specific information.

[josevalim]

@schneems at this point, does it make sense to remove support to the previous mechanism? As it was added just for 4.1?

[schneems]

Life would be slightly easier if we remove the production: "postgres://localhost/foo" case. Having the return value of a key always be one type (a Hash) instead of potentially two types reduces our code paths. Was this feature documented anywhere? What besides taking out the code do we need to do to remove it?