Fix JSON of benchmarks.ipynb, update to nbformat4

When pointing out your benchmarks to someone, I noticed that the benchmarks notebook no longer rendered on the Notebook Viewer.

To address this I

* Deleted a trailing comma from the JSON (causing errors)
* Updated to nbformat4 using `find . -name '*.ipynb' -exec ipython nbconvert --to notebook {} --output {} \;` while on IPython master (not necessary, but good to do)