Testing framework

[darribas]

A couple of bits on here:

• As of 419cd66, we're not testing the book on Windows because of a strange unresolved Windows issue. Ideally, we'd want to be testing on Windows too
• @martinfleis has suggested we might want to set up a testing framework that relies on latest versions always. I think this is one to discuss further. Originally we decided to stick to a fixed environment because the ecosystem was changing very rapidly and we wanted to get the book done... As we're emerging from that and we can imagine the book being a bit more stable, perhaps it's worth revisiting that rule @sjsrey / @ljwolf ?

My original idea to create gds_env is that, unlike for single packages (or at least less so), the dependency structure to have all the libs we need in-sync is such that the probability of one missing something or getting out of sync at any given time (i.e., when you need it for a workshop or event) is large enough. Most of these issues get resolved very quickly but not inmediately, so it's worth having an option you know you can rely on. What do you think? maybe I'm just getting old???