Since i upgraded to Logstash 2.1.1 I am seeing the following when LS starts:
{:timestamp=>"2015-12-11T06:01:55.371000+0000", :message=>"undefined method close' for #Manticore::Client:0x211f70d7", :class=>"NoMethodError", :backtrace=>["/opt/logstash/vendor/bundle/jruby/1.9/gems/elasticsearch-transport-1.0.15/lib/elasticsearch/transport/transport/http/manticore.rb:115:in __close_connections', "org/jruby/RubyArray.java:1613:in each'", "/opt/logstash/vendor/bundle/jruby/1.9/gems/elasticsearch-transport-1.0.15/lib/elasticsearch/transport/transport/http/manticore.rb:115:in __close_connections'", "/opt/logstash/vendor/bundle/jruby/1.9/gems/elasticsearch-transport-1.0.15/lib/elasticsearch/transport/transport/base.rb:99:in __rebuild_connections'", "/opt/logstash/vendor/bundle/jruby/1.9/gems/elasticsearch-transport-1.0.15/lib/elasticsearch/transport/transport/base.rb:77:in reload_connections!'", "/opt/logstash/vendor/bundle/jruby/1.9/gems/logstash-output-elasticsearch-2.2.0-java/lib/logstash/outputs/elasticsearch/http_client.rb:85:in sniff!'", "/opt/logstash/vendor/bundle/jruby/1.9/gems/logstash-output-elasticsearch-2.2.0-java/lib/logstash/outputs/elasticsearch/http_client.rb:73:in start_sniffing!'", "org/jruby/ext/thread/Mutex.java:149:in synchronize'", "/opt/logstash/vendor/bundle/jruby/1.9/gems/logstash-output-elasticsearch-2.2.0-java/lib/logstash/outputs/elasticsearch/http_client.rb:73:in start_sniffing!'", "org/jruby/RubyKernel.java:1479:in `loop'", "/opt/logstash/vendor/bundle/jruby/1.9/gems/logstash-output-elasticsearch-2.2.0-java/lib/logstash/outputs/elasticsearch/http_client.rb:72:in start_sniffing!'"], :level=>:error, :file=>"logstash/outputs/elasticsearch/http_client.rb", :line=>"89", :method=>"sniff!"}
ES is 2.1.0 on the other end.