Official Roundcube support in cPanel has been added in their EDGE builds as of today.
That means we will be seeing it soon in the STABLE, CURRENT, and RELEASE builds very soon. I suggest waiting a short while for this to be deployed as i do not recommend running EDGE builds on a production dedicated server or VPS.
Other good news is that they have made some enhancements in the underlying code for cpanel VPS which should result in 10-40% more efficiency in the memory consumption department, which has always been a problem with cPanel VPS.
Those changes will also be pushed down the line here soon.