SAAS-547: Refactor upgrade procedure in 2.5.x-installer branch

* Start using "schema_version" key instead of "system_version"
* Refactored the upgrade code a bit and make it be automatically invoked
  from the Zend bootstrap if a schema update is needed.
* Fixed error logging in certain cases
* Fixed an exception in Show related to image_path

airtime_mvc/application/models/Show.php

@@ -1232,6 +1232,7 @@ SELECT si.starts AS start_timestamp,
        si.id AS instance_id,
        si.record,
        s.url,
+       s.image_path,
        starts,
        ends
 FROM cc_show_instances si