Fix gentoolkit import pth in portage_utils.py.

Fix gentoolkit import pth in portage_utils.py.

gentoolkit since 0.3.0 has been installed into the usual site-packages
directory, so there shouldn't be any need to monkey with sys.path.

If the user is running an older version that is under
/usr/lib/gentoolkit, we fall back to that.  Appending it to sys.path
(vs. prepending) allows us to override its location with PYTHONPATH:

  PYTHONPATH=".:$HOME/src/gentoolkit/pym" nosetests tests/test_portage_utils.py