Update "Tested up to" in readme.txt file

Resolves issue #2

Author: wpscholar

readme.txt

@@ -4,7 +4,7 @@ Donate link: https://www.paypal.me/wpscholar
 Tags: shortcode
 Requires PHP: 5.6
 Requires at least: 3.2
-Tested up to: 5.1
+Tested up to: 5.4
 Stable tag: 1.0.1
 License: GPLv3
 License URI: http://www.gnu.org/licenses/gpl-3.0.html
@@ -84,4 +84,4 @@ Note: You must have the 'manage_options' capability to see the 'Shortcodes' menu
 == Upgrade Notice ==
 
 = 1.0.1 =
-* Tested in WordPress version 5.1
+* Tested in WordPress version 5.1