Drush from GitHub stopped working
Drush now uses composer for managing external libraries.  You can read about this in the GitHub issue.

If you don't have composer installed, you can follow the instructions on its project page.  Under Linux or OSX, you can do

$ curl -sS https://getcomposer.org/installer | php
$ mv composer.phar /usr/local/bin/composer


which will install it globally.

Once you have done this, you can

$ cd /path/to/your/git/clone/of/drush
$ composer install


This will download all of the external depencencies, and drush should start working.

See the drush project page for details.