Update .travis.yml to ajust to new release cadence (#228)

Author: jsvd

.travis.yml

@@ -1,12 +1,12 @@
 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=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
+jobs:
+  include:
+    - stage: OSS Distribution Tests
+      env: DISTRIBUTION=oss ELASTIC_STACK_VERSION=8.previous
+    - env: DISTRIBUTION=oss ELASTIC_STACK_VERSION=8.current
+    - env: DISTRIBUTION=oss SNAPSHOT=true ELASTIC_STACK_VERSION=8.previous
+    - env: DISTRIBUTION=oss SNAPSHOT=true ELASTIC_STACK_VERSION=8.current
+    - env: DISTRIBUTION=oss SNAPSHOT=true ELASTIC_STACK_VERSION=8.next
+    - env: DISTRIBUTION=oss SNAPSHOT=true ELASTIC_STACK_VERSION=8.future