pin everything in the docs/environment.yml file

[basnijholt]

The documentation is broken once again after a bokeh update...

Relevant: #169

[basnijholt]

I've executed (from #169):

    git branch -D test_documentation
    git checkout -b test_documentation
    git push -f origin test_documentation

so soon we'll see if it works, check: https://adaptive.readthedocs.io/en/test_documentation/tutorial/tutorial.BalancingLearner.html