updating readme file to show the plugin is tested up to 3.9

git-svn-id: https://plugins.svn.wordpress.org/amazon-revenue-sharing/trunk@895461 b8457f37-d9ea-0310-8a92-e5e31aec5664
changed tested up to from 3.8.1 to 3.9

git-svn-id: https://plugins.svn.wordpress.org/amazon-revenue-sharing/trunk@895464 b8457f37-d9ea-0310-8a92-e5e31aec5664