Switch to maybe_unserialize in Job_Agency_Job class constructor

Sometimes when using job-agency, we receive PHP notices expecting type string when unserializing the payload data.  Switched to Wordpress maybe_unserialize.

An example string that triggers the notice is as follows: (PHP 5.4)
a:3:{s:7:"user_id";i:1;s:7:"post_id";i:18800;s:8:"template";s:21:"experience_incomplete";}