Require Sphinx 1.8.5

[jxu]

Bug in Sphinx or RTD theme causes docs to appear wrong.

readthedocs/sphinx_rtd_theme#750

Originally posted by @jxu in #48 (comment)

[llllllllll]

ah, we should ad an extra target like dev (https://github.com/llllllllll/slider/blob/master/setup.py#L39) for docs and pin the version of sphinx and sphinx rtd

[jxu]

actually I tried sphinx==1.8.5 and sphinx-rtd-theme==0.4.2 and still broken for me.
default theme works fine for me so it's a sphinx-rtd-theme issue

[jxu]

For now can we change the theme?

[llllllllll]

Yeah, I like the RTD theme a lot because I mostly consume docs on RTD, but I would be fine changing it to something else that looks nice if there are real issues with it. If find a theme that looks good, I would switch. Maybe we just need to update the numpydoc package which (at least at one time, it may just be part of sphinx now) parsed the numpy-style docstrings.