bring up to date with nbsphinx's compatibility with sphinx 1.5.1

[mpacer]

Same as #493, but for 5.x not master.

#492 (comment) points out that we can update to the latest sphinx so long as we update to the latest nbsphinx.

[takluyver]

5.x doesn't currently have any commits that aren't on master. Our usual practice is to only make maintenance branches when we merge the first thing that won't be in the maintenance releases (i.e. for this branch, when we merge the first 6.0 only change). So I'm going to delete the 5.x branch for now to reduce the maintenance overhead.

[mpacer]

This also was the branch to which our readthedocs pointed. And Matthias pointed out that it is bad to have it pointing to latest anyway since then it is liable to break. That was why I hadn't switched back to latest. This can be solved with a patch release that I'll release tomorrow.

Basically I want to get in the fix for in-memory representations of templates (#491), but that needs another pair of eyes, unless you think the tests look good as is.