As of today, Ubuntu 15.04 (and its official flavours) reach its end of life. There won’t be any more updates or security fixes for this release.
Ubuntu MATE 15.04, released on April 23, 2015, was the first official release of Ubuntu MATE, with 14.04 and 14.10 being unofficial builds.
If you still have any computers running 15.04, the best upgrade path would be to 15.10, which can be done via the Software Updater.
Standard releases have a support window of 9 months.
Long term releases are supported for 5 years. The next one is 16.04 LTS, due April 21, 2016.