Fix ruby version for travis

SpringMT · SpringMT · commit 27be3913c34b · 2017-02-05T00:23:55.000+09:00
diff --git a/.travis.yml b/.travis.yml
@@ -1,8 +1,8 @@
 sudo: false
 language: ruby
 rvm:
-  - 2.4
-  - 2.3
+  - 2.4.0
+  - 2.3.3
   - 2.2
 
 before_install: gem install bundler -v 1.14.3
