Dear David,
Trisquel 7 is based on *buntu 14.04, this means it is a release with
long-term support and perfect for working in a stable environment until 2019.
However, this unfortunately means that you won't be able to install Plasma 5
because the preconditions of the 14.04 release are insufficient.
Only Trisquel 8 will enable you to install Plasma 5.
You can learn more about it at
https://trisquel.info/en/wiki/how-trisquel-made
In GNU/Linux you will generally have to decide between stability and bleeding
edge-software. In regard to this question, Trisquel plays on the serious side
of the game. ;-)
Cheers,
Juergen
