[CI] Test against Ruby 2.5

Author: nicolasleger

.travis.yml

@@ -7,9 +7,11 @@ script: "script/cibuild"
 matrix:
   include:
     # Build with latest ruby
-    - rvm: 2.4
+    - rvm: 2.5
       env: RUBOCOP_TEST="true" RSPEC_TEST="true"
     # Build with older ruby versions
+    - rvm: 2.4
+      env: RUBOCOP_TEST="true" RSPEC_TEST="true"
     - rvm: 2.3.2
       env: RUBOCOP_TEST="false" RSPEC_TEST="true"
     - rvm: 2.2.3