Updated CI configuration

Author: delvedor

.ci/run-elasticsearch.sh

@@ -90,8 +90,6 @@ docker network inspect "$NETWORK_NAME" > /dev/null 2>&1 || docker network create
 environment=($(cat <<-END
   --env node.name=$NODE_NAME
   --env cluster.name=$CLUSTER_NAME
-  --env cluster.initial_master_nodes=$MASTER_NODE_NAME
-  --env discovery.seed_hosts=$MASTER_NODE_NAME
   --env cluster.routing.allocation.disk.threshold_enabled=false
   --env bootstrap.memory_lock=true
   --env node.attr.testattr=test

.ci/test-matrix.yml

@@ -1,6 +1,6 @@
 ---
 ELASTICSEARCH_VERSION:
-  - 6.8.1
+  - 6.8.4
 
 NODE_JS_VERSION:
   - 12