Cannot install thru composer

[maxxer]

$ composer.phar require nemmo/yii2-attachments "*"
 ./composer.json has been updated  
Loading composer repositories with package information
Updating dependencies (including require-dev)
Rea                                                           
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - The requested package nemmo/yii2-attachments could not be found in any version, there may be a typo in the package name.

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.

Installation failed, reverting ./composer.json to its original content.

[maxxer]

works with

composer.phar require nemmo/yii2-attachments "dev-master"

[maxxer]

it would be great if you could update README to reflect the working method