This change bumps gunicorn to version 20.0.4.
It also installs the binary for gunicorn without any suffix, via symlink.
And remove gunicorn_paster; that was removed from the package and moved
into the gunicorn library.
Also, now we need to use the full python3-setuptools package for the
gunicorn executable, as it won't start without it.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>