Fixed #5268: the YAML serializer now uses yaml.safe_dump() instead the plain yaml.dump(); this makes YAML dumps more portable, and also removes the crufty '!!python/unicode' business. Thanks, poelzi.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@6891 bcc190cf-cafb-0310-a4f2-bffc1f526a37