use latest version of ruby

hsbt · hsbt · commit bb8e88cd9e54 · 2017-02-16T14:58:11.000+09:00
diff --git a/.travis.yml b/.travis.yml
@@ -1,6 +1,6 @@
 language: ruby
 rvm:
-  - "2.3.0"
-  - "2.4.0"
+  - 2.3.3
+  - 2.4.0
   - ruby-head
 before_install: gem install bundler -v 1.11.2
