CI doesn't work for jupyter-book==0.6.5 #71

The circleci falls down with the most recent release of jupyter-book, claiming that it cannot find the toc.

Couldn't find a Table of Contents file. To auto-generate one, run

jupyter-book toc .

But there is a toc. Need to work out why this is, for now jupyter-book==0.6.4 is the the requirements.txt