Errors setting up buildbot_travis

I'm setting up a `buildbot_travis` instance to explore various features. I'm following the process outlined in https://github.com/buildbot/buildbot_travis/blob/master/README.rst, i.e. setting up a virtualenv using Python3, then starting the master.
From within the web interface I spawn a "LocalWorker", then set up https://github.com/buildbot/buildbot.git as "git+poller" project. Doing a "force build" eventually results in lots of build failures with errors in the " setup virtualenv" step, specifically
```
Setting up virtualenv sandbox
The path python3.5 (from --python=python3.5) does not exist
program finished with exit code 1
```
I see similar failures for python3.7. Only python3.6 and python2.7 succeed.