BibKnowledge: internationalisation fix

Signed-off-by: Javier Martin Montull <javier.martin.montull@cern.ch>
classifier: registry definition fix

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
installation: fix six version

* Addresses issue with creating forms based on WTForms-Alchemy.
  (addresses #1800)

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>