Fix PHP syntax error in Packages.php, line 35.

Out of the box the postmaster PHP library doesn't work.

Parse error: parse error in postmaster-php/lib/Postmaster/Package.php on line 35

I am running on OSX:

PHP 5.3.15 with Suhosin-Patch (cli) (built: Dec  9 2012 19:32:02)
Copyright (c) 1997-2012 The PHP Group
Zend Engine v2.3.0, Copyright (c) 1998-2012 Zend Technologies

It looks like $headers is trying to be an array, but the syntax is incorrect.