admin-ajax.php not working after update

In WordPress 3.9.2 access to admin-ajax was working for non loggedin users. When I upgraded the script stop working.

I have the following code in functions.php

function theme_name_scripts() {
    wp_enqueue_script('returnval', get_template_directory_uri() . '/library/js/ajax.js', array('jquery'), true );

    wp_localize_script('returnval', 'MyAjax', array(
        // URL to wp-admin/admin-ajax.php to process the request
        'ajaxurl' => admin_url( 'admin-ajax.php' ),

      // generate a nonce with a unique ID "myajax-post-comment-nonce"
        // so that you can check it later when an AJAX request is sent
        'security' => wp_create_nonce( 'str' )
    ));

    wp_enqueue_script( 'jquery' );

}

/*We need this for security */
add_action( 'wp_enqueue_scripts', 'theme_name_scripts' );

function get_images_callback() {
   // check_ajax_referer('my-special-string', 'security' );

        $postID=$_REQUEST['postid'];
        $secImg = get_post_meta($postID, '_cmb_secImg', true );
        $post = get_post($postID);
        $content = apply_filters('the_content', $post->post_content);
        $args = array( 'post_type' => 'attachment', 'posts_per_page' => -1, 'post_status' =>'any', 'meta_query' => array(array('key' => 'mc_selectDisplay','value'=>1)) );
        $attachments = get_posts( $args );

        if ( $attachments ) {
            $imgRecord = 1;
            foreach ( $attachments as $attachment ) {
                $attachUrl=wp_get_attachment_url( $attachment->ID );
                $alt = get_post_meta($attachment->ID, '_wp_attachment_image_alt', true);
                $title = get_post_meta($attachment->ID, 'title', true);
                $lnk= get_post_meta($attachment->ID, 'mc_url', true);
                $description = $attachment->post_content;
                $images[] = array('id'=>$imgRecord,'url'=>$attachUrl,'lnk'=>$lnk,'descr'=>$description,'alt'=>$alt,'title'=>$title);
                $imgRecord = $imgRecord + 1;
            }
        }
    echo json_encode($images);
    die(); // this is required to return a proper result
}

add_action('init', 'ajax_Security');
function ajax_Security(){
    add_action('wp_ajax_get_images', 'get_images_callback');
    add_action('wp_ajax_nopriv_get_images', 'get_images_callback');
}

In My ajax.js i have the following code.

jQuery(document).ready(function($) {

/*Get post id */
  var pstID = 50;

    var data = {
        action: 'get_images',
        postid: pstID,
        security : MyAjax.security,
        dataType: 'JSON'
    };

    /*Preload images */
    $.fn.loadImages= function() {
        this.each(function(){
            $('<img/>')[0].src = this;
        });
    }

    jQuery.ajax({
        url: MyAjax.ajaxurl,
        data:data,
        context: document.body
    }).done(function(data)
      {

          var dbreturn= JSON.parse(data);

          var imgStr="";
          var lstImages="";
          var preloadImg=[];

          /*Loop Thru json data */
          for(var i = 0; i < dbreturn.length; i++) {
              var dbField = dbreturn[i];
              imgStr=imgStr+'<div class="slide"><div class="textslide"><h2>'+dbField.descr+'</h2></div><a href="'+dbField.lnk+'"><img class="img-responsive" src="'+dbField.url+'" alt="'+dbField.alt+'"></a></div>';
              preloadImg[i]=dbField.url;
          }

          /*Preload images */
          $(preloadImg).loadImages();

          /*Append images to slider */
          $("#homeSlider").html(imgStr);

          /*initialize slider */
          jQuery('#homeSlider').cycle({pauseOnHover: true,slides:"> div.slide",pager:"#pager"});

    });

});

This code was working in WordPress 3.9.2