ci: Bump rpm-ostree tag to 2018.8
ci: Bump rpm-ostree tag to 2018.8

2018.7 started failing `test-ucontainer.sh`. I don't have the cycles to
look more deeply into what was going on there, but bumping to 2018.8
fixes it at least. (And of course, it's passing in rpm-ostree too.)

Closes: #1728
Approved by: cgwalters

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree