New doc build failure

Latex problems again. Doc pull requests merged in the last day are #1924 and #1918, by the looks of it.

There's a lot of warnings, ending in:

```
LaTeX Warning: Hyper reference `mpl_toolkits/mplot3d/index:toolkit-mplot3d-inde
x' on page 521 undefined on input line 29783.


LaTeX Warning: File `_images/contourf3d_demo21.png' not found on input line 297
88.


! Package pdftex.def Error: File `_images/contourf3d_demo21.png' not found.

See the pdftex.def package documentation for explanation.
Type  H <return>  for immediate help.
 ...                                              
                                                  
l.29788 ...graphics{_images/contourf3d_demo21.png}
                                                  
? 
! Emergency stop.
 ...                                              
                                                  
l.29788 ...graphics{_images/contourf3d_demo21.png}
                                                  
!  ==> Fatal error occurred, no output PDF file produced!
```

[Build log](https://launchpadlibrarian.net/138514402/buildlog_ubuntu-raring-i386.matplotlib_1.2.0~1%2B6570%2B23~raring1_FAILEDTOBUILD.txt.gz)