As you may know, KDE Plasma 5.3.1 has been recently added to the Kubuntu Backports PPA, permitting the users to easily install the DE on their system:
“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,”
It has not been implemented on Kubuntu 15.04, because KDE Plasma 5.3.1 has been released long after Ubuntu 15.04 got released.
So, to install KDE Plasma 5.3.1 on Kubuntu 15.04, do:
$ sudo add-apt-repository ppa:kubuntu-ppa/backports
$ sudo apt-get update
$ sudo apt-get dist-upgrade