New Sphinx with extra values to unpack

The current version of sphinx installed by conda (v1.2.2) doesn't show the error the newer version produces:  `ValueError:  too many values to unpack` because the elements are not unpacked to the right amount of variables.   This PR (originally in astropy/astropy#2914) add an extra variable to unpack all the elements properly.

In case you are interested, this was introduced in Sphinx by [this commit](https://bitbucket.org/birkenfeld/sphinx/commits/f4743ba9521feb16bedc98f29275868054f01d67)