explicitly default to :file_store in our configuration object

lib/open_project/configuration.rb

@@ -44,7 +44,7 @@ module OpenProject
       'scm_subversion_command'  => nil,
       'disable_browser_cache'   => true,
       # default cache_store is :file_store in production and :memory_store in development
-      'rails_cache_store'       => nil,
+      'rails_cache_store'       => :file_store,
       'cache_expires_in_seconds' => nil,
       'cache_namespace' => nil,
       # use dalli defaults for memcache