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`.

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13487?src=pr) is **84.33%**

> Merging [#13487](https://codecov.io/gh/pydata/pandas/pull/13487?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will not change coverage

``` diff
@@             master     #13487   diff @@
==========================================
  Files           138        138          
  Lines         51083      51083          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          43083      43083          
  Misses         8000       8000          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [20dd17a...9c74612](https://codecov.io/gh/pydata/pandas/compare/20dd17a70063387ccca5f7aca3dbefc904d99457...9c74612a9b31884fde33c2e11f2e8c53e79026a6)
 https://travis-ci.org/pydata/pandas/jobs/138720940
 something still not working, http://pandas-docs.github.io/pandas-docs-travis/style.html

style is blank :<
