test that notebook run

[knaaptime]

the current CI setup tests that the book builds, but does not test that it actually runs, which can be super confusing to users if our canonical tutorials have trouble running. Once #81 is addressed, it would be trivial to make sure that notebooks run using nconvert in thew travis config

[darribas]

+1!!! Checking, it might actually be very straightforward, as the CL tool already allows for execution of notebooks:

notebooks/lib/build.py

Lines 246 to 247 in c86bd66

	if args.test_run:
	_ = test_notebooks(execute=True)

So should be a switch on the CI only. Not sure if they'll be too heavy to run every single time, but at the very least we should enforce the practice of making sure the pass locally.