Update to 6.2.1

Closes #245

README.md

 # Docker ELK stack
 
 [![Join the chat at https://gitter.im/deviantony/docker-elk](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/deviantony/docker-elk?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
-[![Elastic Stack version](https://img.shields.io/badge/ELK-6.1.3-blue.svg?style=flat)](https://github.com/deviantony/docker-elk/issues/244)
+[![Elastic Stack version](https://img.shields.io/badge/ELK-6.2.1-blue.svg?style=flat)](https://github.com/deviantony/docker-elk/issues/245)
 [![Build Status](https://api.travis-ci.org/deviantony/docker-elk.svg?branch=x-pack)](https://travis-ci.org/deviantony/docker-elk)
 
 Run the latest version of the ELK (Elasticsearch, Logstash, Kibana) stack with Docker and Docker Compose.
@@ -122,7 +122,7 @@ Create an index pattern via the Kibana API:
 ```console
 $ curl -XPOST -D- 'http://localhost:5601/api/saved_objects/index-pattern' \
     -H 'Content-Type: application/json' \
-    -H 'kbn-version: 6.1.3' \
+    -H 'kbn-version: 6.2.1' \
     -u kibana:changeme \
     -d '{"attributes":{"title":"logstash-*","timeFieldName":"@timestamp"}}'
 ```

elasticsearch/Dockerfile

 # https://github.com/elastic/elasticsearch-docker
-FROM docker.elastic.co/elasticsearch/elasticsearch-basic:6.1.3
+FROM docker.elastic.co/elasticsearch/elasticsearch-basic:6.2.1
 
 # Add your elasticsearch plugins setup here
 # Example: RUN elasticsearch-plugin install analysis-icu

kibana/Dockerfile

 # https://github.com/elastic/kibana-docker
-FROM docker.elastic.co/kibana/kibana-x-pack:6.1.3
+FROM docker.elastic.co/kibana/kibana-x-pack:6.2.1
 
 # Add your kibana plugins setup here
 # Example: RUN kibana-plugin install <name|url>

logstash/Dockerfile

 # https://github.com/elastic/logstash-docker
-FROM docker.elastic.co/logstash/logstash-x-pack:6.1.3
+FROM docker.elastic.co/logstash/logstash-x-pack:6.2.1
 
 # Add your logstash plugins setup here
 # Example: RUN logstash-plugin install logstash-filter-json