Configure the latest Scrapy stack by default

[Gallaecio]

When scrapinghub.yml does not specify a stack, the default server-side stack is used, which can be quite old, and I imagine should not change to maintain backward compatibility.

However, when shub generates a new scrapinghub.yml file, I see no reason why that file cannot point to a newer stack.

I bet most users do not realize how old the default stack can be compared to the latest one, so this change would, in my opinion, ensure the “least surprise” for users.