Composer fails to download dev-master

[prehfeldt]

I have problems downloading the dev-master of Underscore.php. I've created a new project with nothing else then composer as a requirement and composer fails:

pr@em-webdev-php53:~/projects/test$ ./composer.phar require underscore/Underscore.php
Please provide a version constraint for the underscore/Underscore.php requirement: dev-master
composer.json has been updated
Loading composer repositories with package information
Updating dependencies
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - The requested package underscore/underscore.php dev-master could not be found.

Potential causes:
 - A typo in the package name
 - The package is not available in a stable-enough version according to your minimum-stability setting
   see <https://groups.google.com/d/topic/composer-dev/_g3ASeIFlrc/discussion> for more details.

Read <http://getcomposer.org/doc/articles/troubleshooting.md> for further common problems.

This is my composer.json:

{
    "name": "pr/foobar",
    "authors": [
        {
            "name": "pr",
            "email": "pr@....de"
        }
    ],
    "require": {
        "underscore/Underscore.php": "dev-master"

    }
}

When I change the version to 1.3.1 ist works:

pr@em-webdev-php53:~/projects/test$ ./composer.phar u
Loading composer repositories with package information
Updating dependencies
  - Installing underscore/underscore.php (1.3.1)
    Downloading: 100%

Writing lock file
Generating autoload files

I'm using the latest version of composer.