Import MigrationManager from mongo in mongo backend.

Inside the mongo db backend, use the mongo
MigrationManager. This is hopefully the last reference to
the generic MigrationManager reference on db.util.

mediagoblin/db/mongo/open.py

@@ -18,7 +18,7 @@ import pymongo
 import mongokit
 from paste.deploy.converters import asint
 from mediagoblin.db.mongo import models
-from mediagoblin.db.util import MigrationManager
+from mediagoblin.db.mongo.util import MigrationManager
 
 
 def connect_database_from_config(app_config, use_pymongo=False):

mediagoblin/db/util.py

@@ -14,5 +14,5 @@
 # You should have received a copy of the GNU Affero General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-from mediagoblin.db.mongo.util import (MigrationManager, ObjectId, InvalidId,
+from mediagoblin.db.mongo.util import (ObjectId, InvalidId,
     DESCENDING)