Fix media manager reference in video.

An uninstantiated bare media manager doesn't any more
support ["something"]. You need to do .something.
Fixed for .source_type().

mediagoblin/media_types/video/models.py

@@ -90,7 +90,7 @@ class VideoData(Base):
             return '%s; codecs="%s, %s"' % (
                 mimetype, video_codec, audio_codec)
         else:
-            return video.MEDIA_MANAGER["default_webm_type"]
+            return video.VideoMediaManager.default_webm_type
 
 
 DATA_MODEL = VideoData