Heroku - como habilitar o gd no aplicativo heroku php?

Heroku disse:

As seguintes extensões internas foram construídas “compartilhadas” e podem ser ativadas através do composer.json (nomes de identificadores internos dados entre parênteses):

Mas não dá um exemplo, tentei com o seguinte compositer.json:{ "require": { "gd": "*" } }

Mas quando eugit push heroku master, Eu recebo:

Meu compositer.json:{ "require": { "gd": "*" } }

Mas quando eugit push heroku master, Eu recebo:

-----> Installing dependencies...
       Composer version 1.0.0-alpha9-19-g10401d5 2014-12-09 11:32:02
       Loading composer repositories with package information
       Installing dependencies
       Your requirements could not be resolved to an installable set of packages.

         Problem 1
           - The requested package gd 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.

 !     Push rejected, failed to compile PHP app

Como posso habilitar o gd no heroku ???