Fixed error caused by NoneType object access

I was encountering this error:

    Traceback (most recent call last):
      File "core/main.py", line 65, in run
        out = func(input.inp)
      File "plugins/porktrack.py", line 24, in porktrack
        a = billboard.ChartData('hot-100', date=datetime.strftime(date, "%Y-%m-%d"))
      File "lib/billboard.py", line 45, in __init__
        self.fetchEntries(all=all)
      File "lib/billboard.py", line 74, in fetchEntries
        album = chartInfoSoup.contents[3].string.strip()
    AttributeError: 'NoneType' object has no attribute 'strip'

I fixed it locally, thought I might PR it so others can use it.