DOC: add nbformat for notebook conversion
DOC: add nbformat for notebook conversion

The build [here](https://travis-
ci.org/pydata/pandas/jobs/138633677#L1866) didn't succeed. Had
`nbconvert` in the debs, but not `nbformat`.

Author: Tom Augspurger <tom.augspurger88@gmail.com>

Closes #13487 from TomAugspurger/travis-doc-html-fu and squashes the following commits:

9c74612 [Tom Augspurger] DOC: add nbformat for notebook conversion

MJuddBooth/pandas,Winand/pandas,linebp/pandas,pratapvardhan/pandas,zfrenchee/pandas,GuessWhoSamFoo/pandas,jmmease/pandas,winklerand/pandas,gfyoung/pandas,winklerand/pandas,Winand/pandas,toobaz/pandas,DGrady/pandas,gfyoung/pandas,zfrenchee/pandas,amolkahat/pandas,linebp/pandas,GuessWhoSamFoo/pandas,DGrady/pandas,zfrenchee/pandas,jmmease/pandas,nmartensen/pandas,MJuddBooth/pandas,jreback/pandas,louispotok/pandas,dsm054/pandas,jorisvandenbossche/pandas,TomAugspurger/pandas,MJuddBooth/pandas,rs2/pandas,DGrady/pandas,cbertinato/pandas,linebp/pandas,kdebrab/pandas,louispotok/pandas,pandas-dev/pandas,pandas-dev/pandas,jmmease/pandas,amolkahat/pandas,nmartensen/pandas,Winand/pandas,TomAugspurger/pandas,harisbal/pandas,MJuddBooth/pandas,kdebrab/pandas,jreback/pandas,DGrady/pandas,MJuddBooth/pandas,cython-testbed/pandas,amolkahat/pandas,dsm054/pandas,winklerand/pandas,cython-testbed/pandas,pandas-dev/pandas,winklerand/pandas,louispotok/pandas,datapythonista/pandas,harisbal/pandas,GuessWhoSamFoo/pandas,GuessWhoSamFoo/pandas,Winand/pandas,dsm054/pandas,harisbal/pandas,cbertinato/pandas,gfyoung/pandas,amolkahat/pandas,toobaz/pandas,winklerand/pandas,jreback/pandas,toobaz/pandas,Winand/pandas,dsm054/pandas,jreback/pandas,nmartensen/pandas,TomAugspurger/pandas,pandas-dev/pandas,datapythonista/pandas,jmmease/pandas,winklerand/pandas,linebp/pandas,jreback/pandas,toobaz/pandas,pratapvardhan/pandas,jorisvandenbossche/pandas,nmartensen/pandas,louispotok/pandas,jorisvandenbossche/pandas,DGrady/pandas,pratapvardhan/pandas,GuessWhoSamFoo/pandas,cython-testbed/pandas,Winand/pandas,rs2/pandas,amolkahat/pandas,harisbal/pandas,TomAugspurger/pandas,cbertinato/pandas,datapythonista/pandas,kdebrab/pandas,dsm054/pandas,linebp/pandas,jmmease/pandas,gfyoung/pandas,cbertinato/pandas,kdebrab/pandas,linebp/pandas,gfyoung/pandas,rs2/pandas,toobaz/pandas,kdebrab/pandas,cython-testbed/pandas,cbertinato/pandas,cython-testbed/pandas,pratapvardhan/pandas,DGrady/pandas,nmartensen/pandas,rs2/pandas,jorisvandenbossche/pandas,harisbal/pandas,louispotok/pandas,jmmease/pandas,zfrenchee/pandas,nmartensen/pandas,datapythonista/pandas,pratapvardhan/pandas,zfrenchee/pandas