Updated "tested up to" to 5.1

Author: cristianraiber

readme.txt

@@ -2,7 +2,7 @@
 Contributors: silkalns
 Tags: fancybox, lightbox, jquery, gallery, image, images, photo, photos, picture, pictures, zoom
 Requires at least: 4.6
-Tested up to: 5.3
+Tested up to: 5.1
 Stable tag: 3.1.9
 License: GPLv3 or later
 License URI: http://www.gnu.org/licenses/gpl-3.0.html