Update 1.3.1 release

git-svn-id: https://plugins.svn.wordpress.org/mathjax-latex/trunk@1011318 b8457f37-d9ea-0310-8a92-e5e31aec5664
1.3.1 readme update

git-svn-id: https://plugins.svn.wordpress.org/mathjax-latex/trunk@1011325 b8457f37-d9ea-0310-8a92-e5e31aec5664