ci: Bump flatpak build to F28

Closes: #1607
Approved by: jlebon

.papr.yml

@@ -144,19 +144,19 @@ branches:
     - auto
     - try
 
-context: f27-flatpak
+context: f28-flatpak
 required: true
 
 # This test case wants an "unprivileged container with bubblewrap",
 # which we don't have right now; so just provision a VM and do a
 # docker --privileged run.
 host:
-  distro: fedora/27/atomic
+  distro: fedora/28/atomic
   specs:
     ram: 4096  # build-bundle is a static delta, which needs RAM right now
 
 tests:
-  - docker run --rm --privileged -v $(pwd):/srv/code registry.fedoraproject.org/fedora:27 /bin/sh -c "cd /srv/code && ./ci/flatpak.sh"
+  - docker run --rm --privileged -v $(pwd):/srv/code registry.fedoraproject.org/fedora:28 /bin/sh -c "cd /srv/code && ./ci/flatpak.sh"
 
 artifacts:
   - test-suite.log