BibDocFile: fix version in register_download()
BibDocFile: fix version in register_download()

* Fixes exception raised when the file version passed to
  register_download() is not an integer (this can happen when
  the URL of the file does not explicitly contain the version
  number). (closes #1532)

Reviewed-by: Tibor Simko <c7116e45300b163846dc01f3ed02116e3f8ef7a5@cern.ch>

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