Fix for subscribe merge vars bug introduced in v1.3

git-svn-id: https://plugins.svn.wordpress.org/woocommerce-mailchimp/trunk@886601 b8457f37-d9ea-0310-8a92-e5e31aec5664
Updated readme.txt for 1.3.1

git-svn-id: https://plugins.svn.wordpress.org/woocommerce-mailchimp/trunk@886603 b8457f37-d9ea-0310-8a92-e5e31aec5664