Fix install for Mac OS

[jisantuc]

Overview

This PR makes it so that ./scripts/update works the same on Mac as it does on Linux.
For some reason, setup.py develop fails to install cryptography, while pip install -e .,
which runs setup.py develop as its last step, succeeds.

Also it relaxed the notebook dependency, since apparently ipyleaflet installs 5.0.0 right
now, and paying the cost to install notebook twice is a draaaaaag.

Testing

• be on a Mac
• run ./scripts/update
• observe success

Fixes #5

[hectcastro]

It looks like the updated version of pip is downloading the cryptography wheel, while python setup.py develop is not. Letting modern pip install it via wheel is definitely preferred, so 👍 .