.travis.yml in fluent-plugin-rename-key-0.3.4 vs .travis.yml in fluent-plugin-rename-key-0.4.0

@@ -1,9 +1,10 @@
 language: ruby
 
 rvm:
-  - 2.3.0
+  - 2.3
   - 2.2
+  - 2.1
 
 # to avoid travis-ci issue since 2015-12-25
 before_install:
   - gem update bundler