fix wrong python-dateutil version
The version for python-dateutil (on PyPI) is 1.5 or 2.0.
This has no impact to use 1.5.0 or 2.0.0 if using pip, but when we build
(in my case debian) packages using the setup.py, the dependency on
python-dateutil will be wrong. For Debian, 1.5.0 != 1.5.
So, this fix restores the correct versions for the python-dateutil
dependency.