[FIX] Updater fails to find updates from extension.xml after update from Joomla 2.5.22 to 2.5.24

### Tracker
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_id=8103&tracker_item_id=34082

Since 2.5.23 or https://github.com/joomla/joomla-cms/commit/73f560137fba04cf41394dbb725050bebf964080 Extensions that use more than one `<update>` tag in the update stream can't update via auto updater.
Example: http://vi-solutions.de/updates/visforms/extension.xml
If the last version is on the top (in the first `<update>` tag) or we have only one `<update>` tag all is good and the updates will be find.

This Patch will fix it.