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.

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