Update dateutil to 2.0

Unfortunately, the 2.0 version doesn't work in Python 2.7, and dateutil 1.5
doesn't work in Python 3, so we need to use both versions depending on the
Python version.

Fixes #551.
Don't build 'scipy' in the tests

That should save considerable time on Travis.
Merge pull request #552 from hashdist/dateutil

Update dateutil to 2.0