Fixed nightly core upgrades sometimes running more than once every 24 hours

git-svn-id: https://plugins.svn.wordpress.org/automatic-updater/trunk@747564 b8457f37-d9ea-0310-8a92-e5e31aec5664
Version: 0.9

git-svn-id: https://plugins.svn.wordpress.org/automatic-updater/trunk@747565 b8457f37-d9ea-0310-8a92-e5e31aec5664
Release: 0.9

git-svn-id: https://plugins.svn.wordpress.org/automatic-updater/trunk@747567 b8457f37-d9ea-0310-8a92-e5e31aec5664