Update conda requirements for Pillow

[willingc]

Details

• Project URL: https://readthedocs.org/projects/jupyterhub/
• Build URL (if applicable): https://readthedocs.org/projects/jupyterhub/builds/5242702/
• Read the Docs username (if applicable): willingc

Expected Result

I didn't expect the stable/0.7.2 docs to build. If they did run, I expected the docs to build without failing.

Actual Result

Build is failing since Pillow 3.0.0 is not compatible with Python 3.6. I'm not sure what triggered the build for JupyterHub stable/0.7.2 docs. I triggered the last failure but the other two just appeared 🦇 👻

My recommendation would be allow Pillow to float >=3.0.0 for conda builds to prevent mismatches when there is a newer Python release like 3.6.

Happy to submit the PR if needed.