ci/rpmostree: Bump to 2019.3

The test-ucontainer.sh test in rpmostree 2019.1 is failing for
non-ostree reasons. This is fixed in [1], which is part of 2019.3. I
believe this is still a relevant test of f29 since that's what's shipped
in f29-updates.

1. coreos/rpm-ostree@45b6186

Closes: #1892
Approved by: jlebon

ci/rpmostree.sh

@@ -6,7 +6,7 @@ set -xeuo pipefail
 # Frozen to a tag for now to help predictability; it's
 # also useful to test building *older* versions since
 # that must work.
-RPMOSTREE_TAG=v2019.1
+RPMOSTREE_TAG=v2019.3
 
 dn=$(dirname $0)
 . ${dn}/libpaprci/libbuild.sh