Make sure rack_cache[:verbose] can be set (#96)

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

lib/logstasher/railtie.rb

@@ -30,7 +30,8 @@ class Railtie < Rails::Railtie
         LogStasher.process_config(app.config.logstasher, LOGSTASHER[env].symbolize_keys) if LOGSTASHER.key? env
       end
 
-      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
 
@@ -39,6 +40,10 @@ class Railtie < Rails::Railtie
         LogStasher.setup(config.logstasher) if config.logstasher.enabled
       end
     end
+
+    def rack_cache_hashlike?(app)
+      app.config.action_dispatch.rack_cache && app.config.action_dispatch.rack_cache.respond_to?(:[]=)
+    end
   end
 
   def default_source