Issue with building docs with Sphinx 1.4.0

See https://travis-ci.org/matplotlib/matplotlib/jobs/118997591

```
Warning, treated as error:
WARNING: while setting up extension ['?', 'matplotlib.sphinxext.mathmpl']: node class 'latex_math' is already registered, its visitors will be overridden
```

For the time being I have submitted a pr to limit the Sphinx version to the 1.3 series for the time being see #6234
