[common] Fix Django 1.10 compatibility warning

beat/web/common/storage.py

@@ -38,7 +38,7 @@ class OverwriteStorage(FileSystemStorage):
     def __init__(self, *args, **kwargs):
         super(OverwriteStorage, self).__init__(*args, **kwargs)
 
-    def get_available_name(self, name):
+    def get_available_name(self, name, max_length=None):
         # If the filename already exists, remove it
         if self.exists(name): self.delete(name)
         return name