Fix refresh error when album and track selected.

When you have album open and select album and first track and press
Ctrl-R, you get an error when it tries to run load method on track.
Merge pull request #239 from Sophist-UK/sophist_fix_error_refresh_album_plus_track

Fix refresh error when album and track selected.

Traceback (most recent call last):
  File "./picard/ui/mainwindow.py", line 724, in refresh
    self.tagger.refresh(self.selected_objects)
  File "./picard/tagger.py", line 540, in refresh
    obj.load(priority=True, refresh=True)
AttributeError: 'Track' object has no attribute 'load'