Arvados

I wonder if the real fix here is for arv-run to create jobs that specify arvados_sdk_version: master as a runtime constraint? That's not perfect either, but if it's going to use master's run-command to dispatch the job, it seems like it should specify an SDK version to match, or else problems like this will keep coming up.

The downside to this approach is that arv-run will fail if the Docker image doesn't include virtualenv.