Fix log statment -- Use __mod__ over format style (#190)

Author: marojenka

stdimage/models.py

@@ -62,10 +62,10 @@ def render_variation(cls, file_name, variation, replace=False,
         if storage.exists(variation_name):
             if replace:
                 storage.delete(variation_name)
-                logger.info('File "{}" already exists and has been replaced.',
+                logger.info('File "%s" already exists and has been replaced.',
                             variation_name)
             else:
-                logger.info('File "{}" already exists.', variation_name)
+                logger.info('File "%s" already exists.', variation_name)
                 return variation_name
 
         ImageFile.LOAD_TRUNCATED_IMAGES = True