Update readme, update minimum required version

git-svn-id: https://plugins.svn.wordpress.org/megamenu/trunk@943914 b8457f37-d9ea-0310-8a92-e5e31aec5664
Bump stable tag

git-svn-id: https://plugins.svn.wordpress.org/megamenu/trunk@943916 b8457f37-d9ea-0310-8a92-e5e31aec5664