.travis.yml in concurrent_worker-0.3.3 vs .travis.yml in concurrent_worker-0.4.0

@@ -1,7 +1,10 @@
 ---
 sudo: false
 language: ruby
 cache: bundler
 rvm:
+  - 2.6
+  - 2.5
+  - 2.4
   - 2.3.1
 before_install: gem install bundler -v 2.0.1