Bump license tag in readme.txt

Author: unfulvio

readme.txt

@@ -6,8 +6,8 @@ Requires at least: 3.6.0
 Requires PHP: 5.6
 Tested up to: 5.2.1
 Stable tag: 1.5.4
-License: GPLv2 or later
-License URI: http://www.gnu.org/licenses/gpl-2.0.html
+License: GPLv3 or later
+License URI: http://www.gnu.org/licenses/gpl-3.0.html
 
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