Small fix for DB handler.

Fix this error:

    Traceback (most recent call last):
      File "/home/pouwelse/GITHUB/tribler/Tribler/Main/vwxGUI/SearchGridManager.py",
    line 870, in updateProgressInDB
        self.mypref_db.updateProgressByHash(id, progress)
      File "/home/pouwelse/GITHUB/tribler/Tribler/Core/CacheDB/SqliteCacheDBHandler.py",
    line 1798, in updateProgressByHash
        torrent_id = self.getTorrentIDRoot(hash)
    AttributeError: MyPreferenceDBHandler instance has no attribute
    'getTorrentIDRoot'