Fails to get branches with launchpadlib 1.9.7

[dobey]

With the new launchpadlib, tarmac is failing to find branches. This appears to be due to the Launchpad object's __init__ API breaking in the new launchpadlib, and so tarmac ends up connecting to the default staging server, where many of the branches don't actually exist in launchpad's db. It seems some new positional arguments were added to the __init__, and tarmac was relying on the position of keyword arguments prior; so both ends broke. According to Leonard's e-mail, we should switch to using the Launchpad.login_with() method for creating the launchpad object.