Use a memory cache store in development

In commit 574133a5 we configured the development to use Dalli to cache
pages. However, cache is usually disabled in the development
environment.

When we upgraded to Rails 5 in commit eb36b7e2, we configured the
development environment to enable caching (using a memory store) when a
certain file is present, and to disable it when it's not. This
configuration makes more sense IMHO, and it was being overwritten by the
one previously mentioned.

After this change, using memcached is no longer required in the
development environment and the `DalliError: No server available` error
message is gone.

config/environments/development.rb

@@ -60,7 +60,6 @@ Rails.application.configure do
   # Raises error for missing translations
   # config.action_view.raise_on_missing_translations = true
 
-  config.cache_store = :dalli_store
   config.action_mailer.preview_path = "#{Rails.root}/spec/mailers/previews"
 
   config.after_initialize do