[CI] Update to test against Elasticsearch 6.4

Author: estolfo

Rakefile

@@ -70,10 +70,10 @@ namespace :test do
             --env "cluster.routing.allocation.disk.threshold_enabled=false" \
             --publish 9250:9200 \
             --rm \
-            docker.elastic.co/elasticsearch/elasticsearch:6.3.0
+            docker.elastic.co/elasticsearch/elasticsearch:6.4.0
       COMMAND
       require 'elasticsearch/extensions/test/cluster'
-      Elasticsearch::Extensions::Test::Cluster::Cluster.new(version: '6.3.0',
+      Elasticsearch::Extensions::Test::Cluster::Cluster.new(version: '6.4.0',
                                                             number_of_nodes: 1).wait_for_green
     rescue
     end

elasticsearch-rails/lib/rails/templates/01-basic.rb

@@ -37,7 +37,7 @@
     --env "cluster.name=elasticsearch-rails" \
     --env "cluster.routing.allocation.disk.threshold_enabled=false" \
     --rm \
-    docker.elastic.co/elasticsearch/elasticsearch-oss:6.3.0
+    docker.elastic.co/elasticsearch/elasticsearch-oss:6.4.0
 CMD
 
 begin