Test PyPI pre-releases but allow for failures (#423)
Test PyPI pre-releases but allow for failures (#423)

Using `python setup.py install` will install PyPI pre-releases of
packages while `pip install -r requirements.txt` won't do that. This
lead to CI tests failing as reported in #421. This commit updates the CI
build setup to run two parallell tests, one where we don't use
pre-releases and one where we do but allow them to fail and won't wait
on the run to complete either.

ref: https://github.com/jupyterhub/kubespawner/issues/370#issuecomment-555622221
Fixes #421
kubeflow/fairing,kubeflow/fairing