Remove non available stack release versions from integration CI tests

Author: alexcams

.travis.yml

@@ -7,9 +7,7 @@ jobs:
       env: INTEGRATION=true SNAPSHOT=true LOG_LEVEL=info ELASTIC_STACK_VERSION=7.current
     - env: INTEGRATION=true SNAPSHOT=true LOG_LEVEL=info ELASTIC_STACK_VERSION=8.previous
     - env: INTEGRATION=true SNAPSHOT=true LOG_LEVEL=info ELASTIC_STACK_VERSION=8.current
-    - env: INTEGRATION=true SNAPSHOT=true LOG_LEVEL=info ELASTIC_STACK_VERSION=8.next
     - env: INTEGRATION=true SNAPSHOT=false LOG_LEVEL=info ELASTIC_STACK_VERSION=9.current
-    - env: INTEGRATION=true SNAPSHOT=true LOG_LEVEL=info ELASTIC_STACK_VERSION=9.next
     - env: INTEGRATION=true SNAPSHOT=true ELASTIC_PASSWORD=password ELASTIC_SECURITY_ENABLED=true LOG_LEVEL=info ELASTIC_STACK_VERSION=7.current
     - env: INTEGRATION=true SNAPSHOT=true ELASTIC_PASSWORD=password ELASTIC_SECURITY_ENABLED=true LOG_LEVEL=info ELASTIC_STACK_VERSION=8.current
     - stage: "Secure Integration Tests"