Fix installation of module 'future' in Python 3

[svanoort]

Can't go in main dependencies because it breaks python 2.6 due to inclusion of argsparse sigh.

Maintaining dual 2/3 support is ugly.

Nasty hack options:

• postinstall script that is run when installed by pip?

[svanoort]

It appears to work now, when installed from PyPI main (manually tested by 'pip install pyresttest' on python:3.4, 3.5 docker images).