Adding missing requirements to binder-examples repo

Hi
I have successfully a local binder instance for my organization, and while demoing binder internally I noticed some notebooks given as example cannot be re run without errors, because the requirements.txt does not contain all the dependencies.
I would like to fix this for at least for the binder-examples/requirements repo and I opened this PR:

What do you think, is keeping the requirements of the notebooks shared as examples worthy ?