After failing with pip and easy_install I came across this post again:

https://forum.zato.io/t/what-is-the-canonical-way-to-add-extra-python-packages-to-my-zato-server/580?source_topic_id=586

There is another way, that of updating versions.cfg and buildout.cfg files but if you are more familiar with pip than buildout then pip install package-name is perfectly fine.

So it turns out it is as easy as:

nano ~/current/versions.cfg
update the version from 0.2.0 to 0.8.0 then run
cd ~/current
buildout

And buildout will do it’s magic.