.travis.yml in slack-ruby-client-0.12.0 vs .travis.yml in slack-ruby-client-0.13.0

@@ -16,9 +16,11 @@
         - bundle exec danger
     - rvm: 2.4.1
       env: CONCURRENCY=celluloid-io
     - rvm: 2.4.1
       env: CONCURRENCY=faye-websocket
+    - rvm: 2.5
+      env: CONCURRENCY=async-websocket
   allow_failures:
     - rvm: ruby-head
     - rvm: jruby-head
 
 git: