Require yaml explicitly on #database_configuration

This is something I've found while bootstrapping a new app with rails 4.1.0.rc2 and sequel-rails. Although ```#database_configuration``` is intended to be used by AR (which already [requires yaml](https://github.com/rails/rails/blob/master/activerecord/lib/active_record/base.rb#L1)), it would be a good idea to let other orm adapters to use it (sequel-rails already [uses it](https://github.com/TalentBox/sequel-rails/blob/master/lib/sequel_rails/railtie.rb#L67)). So IMHO I think it's responsibility of this method to require the necessary dependencies.

It seems that sequel-rails works fine until 4.1.0.beta2, from rc1 it raises ```uninitialized constant Rails::Application::Configuration::Psych (NameError)```. After a couple of hours comparing the [changes between beta2 and rc1](https://gist.github.com/planas/9858812) I haven't found the specific commit that makes it fail, but I guess that was another gem requiring yaml at some point (probably i18n).