.travis.yml in howitzer-2.1.1 vs .travis.yml in howitzer-2.2.0

@@ -1,7 +1,8 @@
 language: ruby
 before_install:
   - gem install bundler
 rvm:
-  - 2.2.8
-  - 2.3.5
-  - 2.4.2
+  - 2.3.8
+  - 2.4.9
+  - 2.5.7
+  - 2.6.5