Use the `$current_site` global instead of `wpmu_current_site()`, which is being removed in trunk

See http://irclogs.wordpress.org/chanlog.php?channel=wordpress-dev&day=2014-03-04&sort=asc#m803847
run functional tests against WP 3.9-beta1
Merge pull request #1052 from wp-cli/fix-1052

New site creation broken against trunk