Updated the database version check code.

 * Updated version detection to fail if any of the checks for a version fail (previously if even one passed the version was selected).
 * Updated the system checks to be able to detect database version 1.6.0 
 * Updated the system checks to be able to detect database version 1.5.2 (Beta 2 based detection)
 * Removed ADD COLUMN and INSERT INTO statements from the 1.6.0 upgrade script for items already added by 1.5.2 (Beta 2 based)

<sub>**Note:** Without the first two changes, the Zen Cart 1.6.0 "upgrade" code was not performing a database upgrade when a Zen Cart 1.5.2 database was present. The upgrade process was tested against databases from "clean" Zen Cart installations of 1.5.1, 1.5.2 BETA 2, and 1.6.0 (pre-release).</sub>