Landing bot job changed

[Martin Packman]

I have switched the jenkins landing bot to a new job, as part of the
work on getting it faster and more reliable. If you want to check back
on old results, they are preserved for now under a different job name:

<http://juju-ci.vapour.ws:8080/job/github-merge-juju-old/>

I'm actually having second thoughts in writing this, maybe just
renaming the job to keep the links in existing prs correct would have
been better. I can still do that if anyone has strong opinions.

Martin