Update .travis.yml

Author: jsvd

.travis.yml

@@ -1,2 +1,10 @@
 import:
   - logstash-plugins/.ci:travis/[email protected]
+
+jobs:
+  exclude:
+  - env: ELASTIC_STACK_VERSION=7.current
+  - env: SNAPSHOT=true ELASTIC_STACK_VERSION=7.current
+  include:
+  - env: ELASTIC_STACK_VERSION=7.current RUFUS_SCHEDULER_VERSION=3.0.9 SNAPSHOT=true
+  - env: ELASTIC_STACK_VERSION=7.current RUFUS_SCHEDULER_VERSION=3.0.9