Add 4.0 to test matrix

Author: UnderGreen

.travis.yml

@@ -170,6 +170,13 @@ jobs:
         - echo "$DOCKER_PASSWORD" | docker login -u "$DOCKER_USERNAME" --password-stdin
         - molecule test
         - AUTH_STATE=enabled molecule test
+    - stage: "MongoDB 4.0"
+      env:
+        - MONGODB_VERSION=4.0
+      script:
+        - echo "$DOCKER_PASSWORD" | docker login -u "$DOCKER_USERNAME" --password-stdin
+        - molecule test
+        - AUTH_STATE=enabled molecule test
 
 notifications:
   webhooks: https://galaxy.ansible.com/api/v1/notifications/