.travis.yml in kindlegen-3.0.3 vs .travis.yml in kindlegen-3.0.4

@@ -1,9 +1,10 @@
 language: ruby
 sudo: false
 cache: bundler
 rvm:
-  - 2.1.10
-  - 2.2.6
-  - 2.3.3
+  - 2.4.7
+  - 2.5.7
+  - 2.6.5
+  - ruby-head
 # NOTE the build step is required to run the tests
 script: bundle exec rake build test