.travis.yml in kindlegen-2.9.7 vs .travis.yml in kindlegen-3.0.0

@@ -1,8 +1,9 @@
 language: ruby
 sudo: false
 cache: bundler
 rvm:
-  - 2.1.7
-  - 2.2.3
+  - 2.1.10
+  - 2.2.6
+  - 2.3.3
 # NOTE the build step is required to run the tests
 script: bundle exec rake build test