Make sure rack_cache[:verbose] can be set

This exploded for me.
Copied a commit from lograge which solved the same issue:
roidrage/lograge#103

lib/logstasher/railtie.rb

@@ -8,7 +8,7 @@ class Railtie < Rails::Railtie
     config.logstasher.enabled = false
 
     initializer :logstasher, :before => :load_config_initializers do |app|
-      app.config.action_dispatch.rack_cache[:verbose] = false if app.config.action_dispatch.rack_cache
+      app.config.action_dispatch.rack_cache[:verbose] = false if rack_cache_hashlike?(app)
       LogStasher.setup_before(app.config.logstasher) if app.config.logstasher.enabled
     end
 
@@ -17,5 +17,9 @@ class Railtie < Rails::Railtie
         LogStasher.setup(config.logstasher) if config.logstasher.enabled
       end
     end
+
+    def self.rack_cache_hashlike?(app)
+      app.config.action_dispatch.rack_cache && app.config.action_dispatch.rack_cache.respond_to?(:[]=)
+    end
   end
 end