Fix bug which is causing notebook_test to fail

[gsarma]

With @travs:

Once fixed, remove the skip decorator.

[rgerkin]

@gsarma If you want to try this again, I've updated notebook_test in 03afb30 to first auto-generate a script from the current IPython notebook (the only thing that needs to be under version control), and then run that script. I suspect it will fail due to some unanticipated dependency, but it would be good to figure that out.