v4.20 Released fixing minimum ubuntu version check

Written for Keep.Awake by DanglingPointer on 2020-05-19

Logic used to check the minimum Ubuntu version has been updated. Used to be 14.04, it is now 16.04.

Use KeepAwake version 4.15 for Ubuntu 14.04 or for all other distributions that don't use Mutter Gnome but use Gnome with X.