Fix the detection of republican month names

[Changaco]

Traceback (most recent call last):
  File "legi/tar2sqlite.py", line 575, in <module>
    main()
  File "legi/tar2sqlite.py", line 557, in main
    n_anomalies = detect_anomalies(db, f)
  File "legi/anomalies.py", line 233, in detect_anomalies
    anomalies_textes_versions(db, err)
  File "legi/anomalies.py", line 162, in anomalies_textes_versions
    d1, endpos1 = parse_titre(titre, anomaly_cb('titre'), strict=True)
  File "legi/titles.py", line 121, in parse_titre
    groups['date'], groups['calendar'] = convert_date_to_iso(
  File "legi/fr_calendar.py", line 58, in convert_date_to_iso
    mois = MOIS_GREG_MAP[strip_down(mois)]
KeyError: 'pluviose'