Upgrade buildbot to latest version

We're on 0.8.8, latest in the Ubuntu Xenial repo is 0.8.12, we should at least upgrade to that.

We have issues with 0.8.8 because it is not fully compatible with the version of SQLAlchemy we have installed (python-sqlalchemy 1.0.11), 0.8.12 is compatible.

Even better would be to upgrade to the latest version from http://buildbot.net/ (currently 0.9.6) but this should be done on a different host, set up in parallel so that we don't have downtime, and then we switch over when it's ready.