Update .travis.yml to ajust to new release cadence

This commit updates the travis matrix to use the updated release cadence.

Author: jsvd

.travis.yml

@@ -2,11 +2,21 @@ import:
   - logstash-plugins/.ci:travis/[email protected]
 
 env:
-  - DISTRIBUTION=default ELASTIC_STACK_VERSION=8.x
-  - DISTRIBUTION=oss ELASTIC_STACK_VERSION=8.x
-  - DISTRIBUTION=default ELASTIC_STACK_VERSION=8.x SNAPSHOT=true
-  - DISTRIBUTION=oss ELASTIC_STACK_VERSION=8.x SNAPSHOT=true
+  - DISTRIBUTION=default ELASTIC_STACK_VERSION=8.previous
+  - DISTRIBUTION=default ELASTIC_STACK_VERSION=8.previous SNAPSHOT=true
+  - DISTRIBUTION=default ELASTIC_STACK_VERSION=8.current
+  - DISTRIBUTION=default ELASTIC_STACK_VERSION=8.current SNAPSHOT=true
+  - DISTRIBUTION=default ELASTIC_STACK_VERSION=8.next
+  - DISTRIBUTION=default ELASTIC_STACK_VERSION=8.next SNAPSHOT=true
+  - DISTRIBUTION=default ELASTIC_STACK_VERSION=8.future
+  - DISTRIBUTION=default ELASTIC_STACK_VERSION=8.future SNAPSHOT=true
   - DISTRIBUTION=default ELASTIC_STACK_VERSION=7.x
-  - DISTRIBUTION=oss ELASTIC_STACK_VERSION=7.x
   - DISTRIBUTION=default ELASTIC_STACK_VERSION=7.x SNAPSHOT=true
-  - DISTRIBUTION=oss ELASTIC_STACK_VERSION=7.x SNAPSHOT=true
+  - DISTRIBUTION=oss ELASTIC_STACK_VERSION=8.previous
+  - DISTRIBUTION=oss ELASTIC_STACK_VERSION=8.previous SNAPSHOT=true
+  - DISTRIBUTION=oss ELASTIC_STACK_VERSION=8.current
+  - DISTRIBUTION=oss ELASTIC_STACK_VERSION=8.current SNAPSHOT=true
+  - DISTRIBUTION=oss ELASTIC_STACK_VERSION=8.next
+  - DISTRIBUTION=oss ELASTIC_STACK_VERSION=8.next SNAPSHOT=true
+  - DISTRIBUTION=oss ELASTIC_STACK_VERSION=8.future
+  - DISTRIBUTION=oss ELASTIC_STACK_VERSION=8.future SNAPSHOT=true