Modify OWSLib requirement to prevent conflicts

We will target a specific changeset of OWSLib for the time being, just until
there is a proper release which includes geopython/OWSLib@4b0a62cd.

Basically OWSLib required python-dateutil==2.1, which is intended for
Python 3 and conflicted with the version used by CKAN core.
[#35] Ignore auth when using site_user
[#34] Update lxml requirement

Latest OWSLib introduces changes that require lxml>=2.3.