Update atexit_hook cookbook to be backward compatible

The API has changed, and I did update to pytables 3.1.1 only today. My atexit_hook in my app was broken ...

I then saw that the cookbook has been updated, which is nice (and it's python 3 compatible), but it is not backward compatible with pytables 3.0.0 or 2.x.

I added the missing code to make it backward compatible.

Note: I did not modify the revision number (on top of the page) for the cookbook, the previous commit by @avalentino did also not modify it, so I don't know if this needs to be changed ?
BTW, the page on the website is not up to date and displays the old version: http://www.pytables.org/moin/UserDocuments/AtexitHooks