prevent crash that occurs when changing models.yaml on windows systems

Windows does not support an atomic `os.rename()` operation. This
PR changes it to `os.replace()`, which does the same thing.

ldm/invoke/model_cache.py

@@ -298,7 +298,7 @@ class ModelCache(object):
         with open(tmpfile, 'w') as outfile:
             outfile.write(self.preamble())
             outfile.write(yaml_str)
-        os.rename(tmpfile,config_file_path)
+        os.replace(tmpfile,config_file_path)
 
     def preamble(self):
         '''