CI merge order change for backports

[Harald Sitter]

Applications/15.04 are going to be landed in the backports PPA so we
now created kubuntu_vivid_backports branches for the affected repos
(branched from vivid_archive for kdelibs4 stuff and stable for kf5).

Since this branch is entirely new the CI merge order has been changed
to accomodate it. A test suggests that this should not create any
pointless merges but one never knows, so perhaps keep an eye on the
git logs.

New merge order is:
archive ?-> backports
backports ?-> stable | unstable
archive ?-> stable | unstable
stable ?-> unstable

?-> representing the merge direction only if the source is present,
and | an automatic target switch in case the first is not present.

Previous order was:
archive ?-> stable | unstable
stable ?-> unstable

For the purposes of the automerge backports is equal to a CI branch in
that, if it is not present it will also not be created.

HS