Update version on readme

Author: roykho

readme.txt

@@ -3,7 +3,7 @@ Contributors: automattic, woothemes, mikejolley, akeda, royho, mattyza, slash1an
 Tags: credit card, stripe, woocommerce
 Requires at least: 4.4
 Tested up to: 4.7.2
-Stable tag: 3.1.0
+Stable tag: 3.1.1
 License: GPLv3
 License URI: http://www.gnu.org/licenses/gpl-3.0.html