obfsproxy breaks with old versions of twisted

Debian apparently packages/ships Twisted 12.0.0 (presumably with patches), and I wrote a bunch of the recent code targeting 13.2.0.

Per the jenkins output:

• twisted.internet.error.UnsupportedAddressFamily is missing, requires Twisted 12.1.0.
• twisted.internet.endpoints.HostNameEndpoint is missing, requires Twisted 13.2.0.

Unrelated to this, the jenkins setup is also choking on missing pyyaml.

Thanks to weasel who brought it to my attention.