BibRank: citation flip flop

* Fixes an issue that caused dropping citations on re-indexing during
  the journal catchup phase.

Signed-off-by: Daniel Lovasko <daniel.lovasko@cern.ch>
Co-authored-by: Samuele Kaplun <samuele.kaplun@cern.ch>
Merge pull request #139 from lovasko/citation-flip-flop

BibRank: citation flip flop