Update ELK images

elasticsearch:
- https://github.com/docker-library/elasticsearch/commit/c9aa8ee: Update to 6.8.1
- https://github.com/docker-library/elasticsearch/commit/8ee2df9: Update generated README

logstash:
- https://github.com/docker-library/logstash/commit/a3e3579: Update to 6.8.1
- https://github.com/docker-library/logstash/commit/4f086b1: Update generated README

kibana:
- https://github.com/docker-library/kibana/commit/539ce0a: Update to 6.8.1
- https://github.com/docker-library/kibana/commit/5651d02: Update generated README

library/elasticsearch

@@ -8,6 +8,6 @@ Tags: 7.1.1
 GitCommit: 99f4e9224b542a9810394c8628977795754c70c3
 Directory: 7
 
-Tags: 6.8.0
-GitCommit: 4b7bf0553c99232f27fe8d7e87a2e708849cbdbd
+Tags: 6.8.1
+GitCommit: c9aa8eecee3217d3f0497e29c57b8d822a5eea76
 Directory: 6

library/kibana

@@ -8,6 +8,6 @@ Tags: 7.1.1
 GitCommit: 9267a8b724579791a9ad4a20fd816f03f64e9b64
 Directory: 7
 
-Tags: 6.8.0
-GitCommit: 93ddd7c813613a4609987c2643646a208639965d
+Tags: 6.8.1
+GitCommit: 539ce0a816f7a80b4aac8df4c8805b330991484b
 Directory: 6

library/logstash

@@ -8,6 +8,6 @@ Tags: 7.1.1
 GitCommit: a1c5be368f33e304c843d497b64f8e574e29bf3c
 Directory: 7
 
-Tags: 6.8.0
-GitCommit: 4d095533c8728a7b1dec3c7c96f126fa16f37f03
+Tags: 6.8.1
+GitCommit: a3e35795ecb2890e27611b092f8930c26b8d70c9
 Directory: 6