BibRank: fix redis cache check
BibRank: bug in citation weights storage

* Prevents to store None in the citation weights cache.
Revert "BibRank: fix redis cache check"

This reverts commit adc292c40fe5730479708b541ccc603a13181a2e.
BibCheck: new creation_date check

* Exploits the BibCheck infrastructure to backdate the
  creation date of a record based on the actual content of
  its metadata. In this way tools such as BibSort will be
  able to properly sort records according to their real
  creation.
  (closes #1719)

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>