Update logo / gravatar for organisation

[rgaiacs]

The organisation uses the logo of Jupyter instead of the logo of JupyterHub.

[choldgraf]

+1 from me, my main question is: does JupyterHub have a square logo? If not, what logo would we use?

I just looked and indeed we have this issue about a square logo:

• Create a square JupyterHub logo #526

Personally, I'd be fine with a "good enough but not perfect" logo that could be done very quickly be somebody with a few design skills, or by a community member