Log Message:
Update ruby-rack-cache to 1.6.1

## 1.6.1

  * Revert 'No longer read responses from cache when we already have them'

## 1.6.0

  * Noop backend
  * No longer read responses from cache when we already have them
  * renamed files from entitystore -> entity_store \ 
(metastore/cachecontrol/appengine) and added warns for old ones