Sphinx 2.1.2 breaks our github plugin

[tkell]

Hello @bmcfee!

Via Katie Kinnaird, we now get this:

$ make html
sphinx-build -b html -d _build/doctrees   . _build/html
Running Sphinx v2.1.2

Exception occurred:
  File "/Users/denise.nava/ismir2018-oss-tutorial/doc/sphinxext/github.py", line 149, in setup

    app.info('Initializing GitHub plugin')

AttributeError: 'Sphinx' object has no attribute 'info'

I fixed this by installing Sphinx 1.8.5. We can spec that version in our requirements, or update the github.py in our doc/ directory. What do you think?

[bmcfee]

Huh, I didn't realize that this was in there. Do we need this github extension, or can we get rid of it?

[tkell]

I can't find it being used in our codebase - I vote for removal!