We removed the option to customize the primaryKey (on all databases) for the moment due to some large architectural changes that are needed to handle it properly (we still have hard coded references to the name id and the data type integer / objectID)
For now you can use the uuid plugin for another field but not the primary key.
https://github.com/strapi/strapi/issues/1762
We will eventually add this back in as it was removed around the stable release, but we need to fix the other issues first to make it viable.