[devel at bb.net] Mergeable Buildbot pull requests

[dustin at buildbot.net]

Mergeable pull requests:

https://github.com/buildbot/buildbot/pull/2282 - Drop `changes.pck` support.
https://github.com/buildbot/buildbot/pull/2280 - Update flake8 to version 2.6.
https://github.com/buildbot/buildbot/pull/2268 - Add cascading deletes on foreign keys for builder_masters