Get rid of deprecation warning from aws sdk

Closes #27

Author: mariusgrigaitis

lib/fluent/plugin/out_aws-elasticsearch-service.rb

@@ -80,7 +80,7 @@ def credentials(opts)
       end
       def calback.inspect
         credentials = self.call
-        "#<#{credentials.class.name} access_key_id=#{credentials.access_key_id.inspect}>"
+        credentials.credentials.inspect
       end
       calback
     end