When I implemented the migration code to update the CACHES setting from

the old MemcachedCache to the new PymemcacheCache, I tested the

actual migration implementation, but had just made an assumption about

the code that checks if we need to run it. It turns out this had two

problems--we were skipping one step that needed to check if a settings

update was required, and in the running context of rb-site, the key that

we need to check is forwarded_backend instead of default.

Ran a real rb-site upgrade and saw that my settings_local.py file was

correctly updated.