Fix the plugin does not have a valid header issue on 1st activation.

git-svn-id: https://plugins.svn.wordpress.org/slimjetpack/trunk@775946 b8457f37-d9ea-0310-8a92-e5e31aec5664
Update the readme.txt stable tag

git-svn-id: https://plugins.svn.wordpress.org/slimjetpack/trunk@775947 b8457f37-d9ea-0310-8a92-e5e31aec5664