Update travis config for new test matrix (#214)

This commit updates the travis config to reflect the new release stream for
logstash. In addition to the new test cells we use the upstream docker-setup.sh
script from the upstream `.ci` repo.

.travis.yml

@@ -4,13 +4,15 @@ import:
 before_install:
 - sudo sysctl -w vm.max_map_count=262144 # due ES bootstrap requirements
 
-env:
-- INTEGRATION=false ELASTIC_STACK_VERSION=7.x
-- INTEGRATION=false ELASTIC_STACK_VERSION=8.x
-- INTEGRATION=false ELASTIC_STACK_VERSION=7.x SNAPSHOT=true
-- INTEGRATION=false ELASTIC_STACK_VERSION=8.x SNAPSHOT=true
-- INTEGRATION=true ELASTIC_STACK_VERSION=7.x
-- INTEGRATION=true ELASTIC_STACK_VERSION=7.x SNAPSHOT=true LOG_LEVEL=info
-- INTEGRATION=true ELASTIC_STACK_VERSION=8.x SNAPSHOT=true LOG_LEVEL=info
-- SECURE_INTEGRATION=true INTEGRATION=true ELASTIC_STACK_VERSION=7.x LOG_LEVEL=info
-- SECURE_INTEGRATION=true INTEGRATION=true ELASTIC_STACK_VERSION=7.x ES_SSL_SUPPORTED_PROTOCOLS=TLSv1.3
+jobs:
+  include:
+    - stage: "Integration Tests"
+    - 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=true LOG_LEVEL=info ELASTIC_STACK_VERSION=8.future
+    - stage: "Secure Integration Tests"
+    - env: SECURE_INTEGRATION=true INTEGRATION=true LOG_LEVEL=info ELASTIC_STACK_VERSION=8.current SNAPSHOT=true
+    - env: SECURE_INTEGRATION=true INTEGRATION=true LOG_LEVEL=info ELASTIC_STACK_VERSION=7.current
+    - env: SECURE_INTEGRATION=true INTEGRATION=true LOG_LEVEL=info ELASTIC_STACK_VERSION=7.current ES_SSL_SUPPORTED_PROTOCOLS=TLSv1.3