3.9 compatibility

git-svn-id: https://plugins.svn.wordpress.org/ect-add-to-cart-button/trunk@899260 b8457f37-d9ea-0310-8a92-e5e31aec5664
Readme.txt updated

git-svn-id: https://plugins.svn.wordpress.org/ect-add-to-cart-button/trunk@899263 b8457f37-d9ea-0310-8a92-e5e31aec5664