Attempt to show correct `Gush version` value

After I installed Gush using `composer global require 'gushphp/gush=dev-master'` it shows `Gush version @package_version@`, but when I installed it via `curl -sS http://gushphp.org/installer | php` it shows correct version as `Gush version 1.3.9`.

I don't know how to verify that my fix actually works and don't have time for this now.