BibField: change recid field type to integer

* Fixes problems with fields like `number_of_citations` where
  an integer was needed instead of a string.  (closes #1633)

Signed-off-by: Esteban J. G. Gabancho <esteban.gabancho@cern.ch>
Reviewed-by: Tibor Simko <tibor.simko@cern.ch>