The download folder contains binaries of older versions for Windows, and the source code of the older versions.
To use with Ubuntu, you have to compile from source on your own, respecting the dependencies of other packages. This installation process is described in http://htmlpreview.github.io/?https://github.com/qgis/QGIS/blob/master/doc/INSTALL.html
Another choice is to set up an older version of ubuntu using a virtual box inside your running Ubuntu. Ubuntugis-unstable still has packages of QGIS 2.4 for saucy, and 2.0 for raring, quantal and lucid.