Replace deprecated wpmu_current_site

`wpmu_current_site()` will be deprecated in 3.9. It has been a private function in the past, though still somewhat useful.

The [use of this](https://github.com/danielbachhuber/dictator/blob/58a9680d40bd5d99a0cbccd3a4a2857d48383f28/php/regions/class-network-sites.php#L280) in `Network_Sites::create_site()` should probably be changed to use the `$current_site` global.