bad pinning for dateutil avoids clean pip install

pkg_resources.VersionConflict: (python-dateutil 2.2 (/home/kali/Virtualenvs/leap-bitmask/lib/python2.7/site-packages), Requirement.parse('python-dateutil>=1.4,<2'))

(from redmine: created on 2014-09-18, closed on 2014-10-09, relates #5718 (closed))