Fixed pip version problem: https://github.com/cuckoosandbox/cuckoo/is…

[Qubasa]

…sues/2678

Thanks for contributing! But first: did you read our community guidelines?
https://cuckoo.sh/docs/introduction/community.html

What I have added/changed is:

I swapped one line where a non stable pip api function has been used to a stable api function call:
#2678

Another Issue detailing the problem:
pypa/pip#5243

The goal of my change is:

That cuckoo works with the newest version of pip

[jbremer]

Thanks, this has been manually merged!