BibClassify: compatibility with rdflib < 2.3.2
BibClassify: compatibility with rdflib < 2.3.2

* Make bibclassify run with older rdflib versions too.  Based on changes
  done in 6796ed47bb365c2589b30ed293cbb671981e3640.

inveniosoftware/invenio,inveniosoftware/invenio,tiborsimko/invenio,tiborsimko/invenio