Update jupyterhub launch link

[rabernat]

earth-env-data-science-book/_config.yml

Lines 50 to 53 in c12a2d6

	# Jupyterhub link settings
	use_jupyterhub_button: true # If 'true', display a button that will direct users to a JupyterHub (that you provide)
	jupyterhub_url: 'https://ocean.pangeo.io' # The URL for your JupyterHub. If no URL, use ""
	jupyterhub_interact_text: ocean.pangeo.io # The text that interact buttons will contain.

Point it at the new Columbia jupyerhub.

[rabernat]

This could easily be combined with #11