ci: also test with ruby 2.4

.gitlab-ci.yml

@@ -18,6 +18,15 @@ ruby2-3:
     paths:
       - log/test.log
 
+ruby2-4:
+  image: "ruby:2.4"
+  script:
+    - bundle exec rake test
+  artifacts:
+    when: on_failure
+    paths:
+      - log/test.log
+
 services:
   - couchdb:1.6.1