Another funny thing today – a system running PHP 5.3.8, but no phar support! Strange, considering that the docs say phar support is build in since 5.3.0.
Anyway, the solution is to unpack the vendor.phar file included in ZenMagick releases on a system that does support phar, for example your local dev system.
The way to go could be to include temporarily the following lines to your bootstrap.php file.
(Note: As of 0.9.13, the location of the vendor folder changed to zenmagick/vendor)
$vendorDir = dirname(__FILE__).'/vendor';
$phar = new Phar($vendorDir.'/vendor.phar');