Our workers are failing to start up on the Caltech cluster with
the following error message:

```
/home/emfollow-playground/.local/lib/python3.7/site-packages/celery/utils/imports.py:167: UserWarning: Cannot load celery.commands extension 'gwcelery.tools.flask:FlaskCommand': ImportError("/lib64/libstdc++.so.6: version `CXXABI_1.3.9' not found (required by /cvmfs/ligo-containers.opensciencegrid.org/lscsoft/conda/latest/envs/ligo-py37/lib/python3.7/site-packages/kiwisolver.cpython-37m-x86_64-linux-gnu.so)")
```

Since Healpy contains C++ code, let's try downgrading to a slightly
older version of Healpy.