Use mem_cache_store instead of dalli_store

`dalli_store` is deprecated since dalli 2.7.11.

We can now enable cache_versioning. We didn't enable it when upgrading
to Rails 5.2 because of possible incompatibility with `dalli_store` [1],
even though apparently some the issues were fixed in dalli 2.7.9 and
dalli 2.7.10 [2].

Since using cache versioning makes cache expiration more efficient, and
I'm not sure whether the options we were passing to the dalli store are
valid with memcache store (documentation here is a bit lacking), I'm
just removing the option we used to double the default cache size on
production.

[1] https://www.schneems.com/2018/10/17/cache-invalidation-complexity-rails-52-and-dalli-cache-store
[2] https://github.com/petergoldstein/dalli/blob/master/History.md

config/environments/staging.rb

@@ -57,7 +57,7 @@ Rails.application.configure do
   # config.logger = ActiveSupport::TaggedLogging.new(SyslogLogger.new)
 
   # Use a different cache store in production.
-  config.cache_store = :dalli_store
+  config.cache_store = :mem_cache_store
 
   # Enable serving of images, stylesheets, and JavaScripts from an asset server.
   # config.action_controller.asset_host = "http://assets.example.com"