CC-2596: Upgrade script doesn't recognize existing Airtime 1.7->1.9

- fixed
- The script will print out error and exit upon detecting any versions
prior to 1.8.0:

A version of Airtime older than 1.8.0 detected, please upgrade to 1.8.0 first.
You will then be able to upgrade to 1.9.0 using this installer.

install_minimal/include/AirtimeInstall.php

@@ -68,6 +68,8 @@ class AirtimeInstall
             if (PEAR::isError($version)) {
                 return null;
             }
+            // no version string detected
+            $version = false;
             return $version;
         }
     }