Pin to `jupyterhub~=1.5` for development, update `dockerspawner`, default ref to `HEAD`

[jtpio]

JupyterHub 2.0 was released yesterday: https://blog.jupyter.org/jupyterhub-2-0-7a038715e96d

But TLJH still installs jupyterhub==1.* for now:

https://github.com/jupyterhub/the-littlest-jupyterhub/blob/ad38fad96afe8096e722279da095a0633b5d2c6b/tljh/installer.py#L124

This avoids picking up the new version for now, so they upgrade to 2.0 can be done explicitly.

Other updates:

• [x] Update to dockerspawner~=12.1
• [x] Update node on CI
• [x] Update default ref to HEAD. Fixes #43.

[jtpio]

CI is green.

We now also default to HEAD for the git reference: