Fix load_weights() for Keras versions before 2.2

Improve previous commit to not break on older versions of Keras.

mrcnn/model.py

@@ -2094,7 +2094,13 @@ class MaskRCNN():
         exlude: list of layer names to excluce
         """
         import h5py
-        from keras.engine import saving
+        # Conditional import to support versions of Keras before 2.2
+        # TODO: remove in about 6 months (end of 2018)
+        try:
+            from keras.engine import saving
+        except ImportError:
+            # Keras before 2.2 used the 'topology' namespace.
+            from keras.engine import topology as saving
 
         if exclude:
             by_name = True