BibHarvest: no dependency of BibRecord

* Use of BibRecord module is being removed from harvesting-kit
  and now PosPackage.get_record() returns a xml.dom.minidom
  object instead of a BibRecord object.

Tested-by: Jan Aage Lavik <jan.age.lavik@cern.ch>
Signed-off-by: Georgios Papoutsakis <georgios.papoutsakis@cern.ch>