Unpin sphinx 1.x for doc build

To work around https://github.com/dask/dask-ml/issues/512, we pinned sphinx to 1.x. We'll want to avoid getting too far behind.

Start by locating the Sphinx 1.x pin in the documentation build configuration and read dask/dask-ml#512 to understand the original workaround. Check whether the documentation build succeeds without the pin and confirm that the dependency no longer remains unnecessarily behind.

Written by the indexing model from the issue text.