CI failure on release commits, when we remove doc from table of contents

The release instructions for IPython say to remove the `whatsnew/development` page from the table of contents in the docs before releasing.

However, we've also made Travis fail if there are any warnings building the docs. A page which is not in the TOC creates a warning, so the tests fail on every release commit, and then go back to passing for development. E.g. https://travis-ci.org/ipython/ipython/jobs/363033501