#4 — captcha time periods are incorrectly calculated

collective/captcha/browser/captcha.py has a subtle bug in _generate_words: the intent was to allow a captcha to be used within a 10-minute window but the current time is converted to 5 minute intervals before subtracting for the previous time period in minutes:

        nowish = _TEST_TIME or int(time.time() / 300)
        secret = getUtility(IKeyManager).secret()
        seeds = [sha.new(secret + session + str(nowish)).digest(),
                 sha.new(secret + session + str(nowish - 5)).digest()]

This should be the number of 5-minute intervals ago:

        nowish = _TEST_TIME or int(time.time() / 300)
        secret = getUtility(IKeyManager).secret()
        seeds = [sha.new(secret + session + str(nowish)).digest(),
                 sha.new(secret + session + str(nowish - 1)).digest()]