Update Ajax.php

Got following error on 1.9.0.1:
Recoverable Error: Argument 1 passed to Varien_Data_Form_Element_Abstract::addElement() must be an instance of Varien_Data_Form_Element_Abstract, instance of Lesti_Version_Block_Adminhtml_Data_Form_Element_Version_Ajax given, called in /var/www/lib/Varien/Data/Form/Abstract.php on line 148 and defined  in /var/www/lib/Varien/Data/Form/Element/Abstract.php on line 55

Changing base class to Varien_Data_Form_Element_Abstract did the trick for me.
Merge pull request #1 from adamkarnowka/patch-1

Changed base class in Ajax block.