Fix problems with new discogs_client version

I just noticed that discogs_client changed module structure a little but, so I got this.
```python
** error loading plugin discogs
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/beets/plugins.py", line 193, in load_plugins
    namespace = __import__(modname, None, None)
  File "/usr/lib/python2.7/site-packages/beetsplug/discogs.py", line 20, in <module>
    from discogs_client import DiscogsAPIError, Release, Search
ImportError: cannot import name DiscogsAPIError
```