bump 1.5.1 in readme.txt

was still 1.5.0

Author: unfulvio

readme.txt

@@ -5,7 +5,7 @@ Requires at least: 3.8
 Tested up to: 4.4
 Requires WooCommerce at least: 2.2
 Tested WooCommerce up to: 2.4.10
-Stable tag: 1.5.0
+Stable tag: 1.5.1
 
 This plugin extends WooCommerce by setting sequential order numbers for new orders.