Change to xpra version 0.11.6 as 0.12.5 was not working

Version 0.11.6 was the latest version that I was able to get building
and running on NixOS. Unfortunately my previous commit for 0.12.5 was
an error and incomplete.
Merge pull request #2957 from doublec/xpra

Change to xpra version 0.11.6 as 0.12.5 was not working