Add dependencies for Travis CI testing

I have renamed the PIP requirements file since this is the default name.
I have also added CKAN as a dependency. While this seems odd, it is just
correct since the extension makes use of CKAN library methods. If CKAN
is already installed this requirement is satisfied anyway.

Signed-off-by: Konrad Reiche <konrad.reiche@fokus.fraunhofer.de>