Run tests under ubuntu 18.04.
Run tests under ubuntu 18.04.

I think this should fix some sporadic test failures with "pstree"
failing due to https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=815902

NREL/api-umbrella,NREL/api-umbrella,NREL/api-umbrella,NREL/api-umbrella