Update virtualenv instructions
    
    * Koji can now be installed from PyPI and it will also pull in rpm. This
      needs rpm-devel install on the system. This means we can get kobo from
      PyPI as well.
    * Using print function should make it work for python 3 virtualenvs.
    * Newer we need kobo in 0.6.0 at least, but newer is fine too.
    
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>