Bug 1672865: Update Zend framework to 1.12.20

Only changed the version number.
The upgrade modified a part of Zend framework we dont use.

Change-Id: Ifaa190e0d75823c50966c3c9c8f6d2b8b792d568
behatnotneeded: no change in functionality

htdocs/webservice/libs/zend/README.Mahara

@@ -4,7 +4,7 @@ Zend Web Services in Mahara
 Please see the associated readme_moodle.txt
 
 Website: http://framework.zend.com/
-Version: 1.12.19
+Version: 1.12.20
 
 
 Changes: