add semaphore lock to prevent wp-cron causing double backups

git-svn-id: https://plugins.svn.wordpress.org/updraftplus/trunk@774958 b8457f37-d9ea-0310-8a92-e5e31aec5664
version number bump

git-svn-id: https://plugins.svn.wordpress.org/updraftplus/trunk@774960 b8457f37-d9ea-0310-8a92-e5e31aec5664