correctly specify which txtorcon version we depend on

[david415]

a friendly cypherpunk at the tor-dev meeting here in amsterdam mentioned that txmix doesn't build because the latest txtorcon master branch isn't the version of txtorcon that i used when i wrote txmix. now that txtorcon has updated it's version number we can just pin to that version and fix our code. i will do this soon after i'm not distracted by this meeting.