Xubuntu 15.04 introduced Xfce 4.12 with some new features landing in addition to many bugfixes over Xfce 4.10/4.11 components.
This release has reached End of Life
The downloads are listed here only for archival purposes. Running this release is not recommended under any circumstances.
I understand and accept the risks
The above link will redirect you to old-releases.ubuntu.com, where you will have to navigate to the correct release manually. You have been warned.