Bower has changed their registry URL some time ago and has now announced to deprecate the old URL. Old registry will be disabled next Wednesday. In order for your work not to be affected make sure to update your composer asset plugin to the latest version:
We are very pleased to announce that Yii 2 is now available for public preview at GitHub. This is a major milestone during the Yii 2 development which was started more than two years ago and has since undergone complete rewrite.