Update version to 6.1.0 (#212)

Updated the version of the docker image tags to 6.1.0 and bumped
the version of the ELK badge

* Linked ELK badge to issue #212

The badge link to the pull request was not updated to reflect the
current pull request's number

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-6.0.0-blue.svg?style=flat)](https://github.com/deviantony/docker-elk/issues/196)
+[![Elastic Stack version](https://img.shields.io/badge/ELK-6.1.0-blue.svg?style=flat)](https://github.com/deviantony/docker-elk/issues/212)
 [![Build Status](https://api.travis-ci.org/deviantony/docker-elk.svg?branch=master)](https://travis-ci.org/deviantony/docker-elk)
 
 Run the latest version of the ELK (Elasticsearch, Logstash, Kibana) stack with Docker and Docker Compose.

elasticsearch/Dockerfile

 # https://github.com/elastic/elasticsearch-docker
-FROM docker.elastic.co/elasticsearch/elasticsearch-oss:6.0.0
+FROM docker.elastic.co/elasticsearch/elasticsearch-oss:6.1.0
 
 # 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-oss:6.0.0
+FROM docker.elastic.co/kibana/kibana-oss:6.1.0
 
 # 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-oss:6.0.0
+FROM docker.elastic.co/logstash/logstash-oss:6.1.0
 
 # Add your logstash plugins setup here
 # Example: RUN logstash-plugin install logstash-filter-json