This happens on master and in release 1.27:
[~/www/wiki-27/extensions/Collection]# php ../../composer/composer.phar update
Loading composer repositories with package information
Updating dependencies (including require-dev)
Your requirements could not be resolved to an installable set of packages.
- The requested package mediawiki/mediawiki could not be found in any version, there may be a typo in the package name.
- A typo in the package name
- The package is not available in a stable-enough version according to your minimum-stability setting see https://getcomposer.org/doc/04-schema.md#minimum-stability for more details.
Read https://getcomposer.org/doc/articles/troubleshooting.md for further common problems.