update for python-discid stable

The previous code gives a DeprecationWarning starting with python-discid
0.5.0.
This is now the indended usage for the stable python-discid release.

When discid.read() doesn't throw DiscError, disc.id does exist and is
not None.
Merge pull request #2 from JonnyJD/discid_read

isrcsubmit-cdrdao.py: The DiscId class is deprecated (since python-discid 0.5.0)