Callback function on change event
The #ajax['wrapper'] value should be id attribute, not a class, and without '#'. You can look about this here. The callback function should return a html content, a renderable array (which will replace the 'wrapper' on a call) or array of drupal ajax commands. You can try this and your code should work: 

function MYMODULE_ajax_callback($form, $form_state) {
   $commands[] = ajax_command_alert(t('MYMODULE_ajax_callback fired'));
   return array('#type' =&gt; 'ajax', '#commands' =&gt; $commands);
}