Upgrade repo2docker to get julia image to build
Upgrade repo2docker to get julia image to build

https://app.circleci.com/pipelines/github/berkeley-dsep-infra/datahub/1509/workflows/5c9ebf4d-bcb7-4f87-9786-b4534175c8dc/jobs/17172
reports the following error when trying to build julia image:

```
Step 47/63 : RUN chown ${NB_USER}:${NB_USER} ${REPO_DIR}
 ---> Running in b9d6d39bca17
chown: changing ownership of '/srv/repo': Operation not permitted
```

I suspect this is fixed with
https://github.com/jupyterhub/repo2docker/pull/975,
so we upgrade repo2docker in CircleCI to see if that works.

berkeley-dsep-infra/datahub,berkeley-dsep-infra/datahub,berkeley-dsep-infra/datahub,ryanlovett/datahub,ryanlovett/datahub,ryanlovett/datahub