DOC: disable nbsphinx including requirejs
To fix https://github.com/pandas-dev/pandas-sphinx-theme/issues/25. With their latest release, `nbsphinx` started including require.js by default, which doesn't play nice with the bootstrap.js (and as a result, no javascript at all works). 
This is using an option of `nbsphinx` to not include requirejs (which we don't need, nbsphinx added this for rendering notebooks with plotly, I think)