KDE Plasma 5.3.1 was just announced by the KDE Community a few days ago, and now it has been made available in Kubuntu 15.04 with the help of a single PPA.
Just because KDE Plasma 5.3.1 was released long after the final edition of Kubuntu 15.04 was made available doesn't mean that the users can't take advantage of the newer versions. This is done, like in any other Ubuntu-based system, via a PPA. Even so, the developers still need to put some effort into the porting process.
"Packages for the release of KDE's desktop suite Plasma 5.3.1 are available for Kubuntu 15.04. You can get it from the Kubuntu Backports PPA. Bugs in the packaging should be reported to Kubuntu-ppa on Launchpad," reads the announcement.
Adding the PPA to the system is quite easy. Just enter the following commands into a terminal (you need to be root in order for this to work):
sudo apt-get update
sudo apt-get dist-upgrade