Make vdm 0.11 a PyPI package dependency

I just had to install CKAN in an offline environment. It would have helped if the CKAN dependencies were all non-editable ie. published as packages on PyPI (e.g. [like this](http://stackoverflow.com/a/14447068/114462)).

`vdm` is the only editable dependency left because `vdm==0.11` is not published on PyPI (see https://github.com/okfn/vdm/issues/5 ).