The project uses a couple of open source libraries such as PHPMailer. To avoid having to include them in the repository, we have just switched to Composer to install them.
For regular releases this will not make a difference, all dependencies will be included in the zip and tar files so no extra action is required. If you are downloading the source from GitHub, however, you will need to install these dependencies yourself. To do that, you can run the following command from the command line after downloading the source:
php composer.phar install