update vips.rb to the new vips-7.42

I have a packaging question before this is merged.

7.42 includes a new Python interface which works with python2 and python3. This relies on python2 having the "future" module installed with something like "pip install future". 

What would be the best way to express this requirement in my formula? I've searched, but can't find a formula which has this kind of feature. I'm probably being dumb.