Change to Ubuntu Focal (20.04) as build environment on Travis Ci, cleanup configuration and enable builds on arm64, too.
Change to Ubuntu Focal (20.04) as build environment on Travis Ci,
cleanup configuration and enable builds on arm64, too.

gridcf/gct,gridcf/gct,gridcf/gct,gridcf/gct,gridcf/gct,gridcf/gct