warn if python-musicbrainz2 is too old

[JonnyJD]

0.7.4 is needed for the user agent string (otherwise we have sporadic server errors)

0.7.3 is needed AUTH DIGEST, which is necessary for NGS?
See: http://bugs.musicbrainz.org/ticket/5905

Mineo did also something similar:
mineo/mbstuff@f3f1fa9#isrcsubmit-cdrdao.py

[JonnyJD]

b644c2c also fixed this issue, together with #2