reverted back to 1.6.4 README.txt

git-svn-id: https://plugins.svn.wordpress.org/custom-facebook-feed/trunk@797942 b8457f37-d9ea-0310-8a92-e5e31aec5664
1.6.6

git-svn-id: https://plugins.svn.wordpress.org/custom-facebook-feed/trunk@797943 b8457f37-d9ea-0310-8a92-e5e31aec5664