For a very long time, the idea of the x.5 series of a Joomla release is the stable and official version that you should be using and for a long time people would have been waiting for this release before upgrading, migrating or developing for the platform.
This is no longer the case and had not been since ~ November 2013.
You no longer have to use and wait for the x.5 release of Joomla. You just use the latest version of Joomla that you can download.
No longer are there two different or multiple versions, e.g 2.5.x and 3.1.x, of Joomla that you can download but only the one latest version.