Add support for Python2.7

[whart222]

Some packages, like Gurobi, only support Python 2.7. Can we automate the creation of packages for Python 2.7?

[astrojuanlu]

We initially sticked to Python 3.5 to iterate faster, because debugging AppVeyor builds is a pain in the neck and locally building Windows extensions is easier in Python 3.5 (see for instance this lengthy post about the changes that took place in the last version).

Apart from that, there's no reason not to automate the creation of packages for Python 2.7 too (and while we're at it Python 3.4, in case anybody needs it).

[whart222]

That makes sense.