CI: latest ipython version for doc build
See https://github.com/pydata/pandas/pull/12002#issuecomment-174550631

Doc builds are failing, testing if using the latest `ipython` version solves it.

lgtm. The build _looked_ like it succeded.
 @jorisvandenbossche shall we merge this?
 yes!
 ok, merging now
 dev docs built! looks good.

lots of warnings in the build of course....

and did API reference! awesome!
 > lots of warnings in the build of course....

Yes, there is an issue with our custom accessor documenters that are not working anymore with the latest sphinx (that I needed for getting this run on travis). So need to investigate that ..

Apart from that, we normally don't have that many warnings! :-)
