赫罗库说：
以下内置扩展是“共享”构建的，可以通过composer.json（括号中给出的内部标识符名称）启用：
但它没有给出一个示例，我尝试了以下composer.json： { "require": { "gd": "*" } } 但是当我 git push heroku master ，我得到：
my composer.json： { "require": { "gd": "*" } } 但是当我 git push heroku master ，我得到：

-----> 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

如何在heroku上启用gd？？？