Update .travis.yml

Author: haruyama-makoto

.travis.yml

@@ -1,10 +1,10 @@
 sudo: false
 language: ruby
 rvm:
-  - 2.6.0
-  - 2.5.3
-  - 2.4.5
-  - 2.3.8
+  - 2.6
+  - 2.5
+  - 2.4
+  - 2.3
   - 2.2
 
 before_install: gem install bundler -v 1.14.3