make existing YAML dependency explicit

(the ActiveRecord tests always have YAML loaded already,
but what about when you're not running the ActiveRecord
tests?)

spec/support/active_record/database_setup.rb

@@ -1,3 +1,5 @@
+require 'yaml'
+
 def db_config
   config_path = 'db/config.yml'
   @db_config ||= YAML.load(IO.read(config_path))