Use Ruby 2.3.8, 2.4.5, and 2.5.3

Author: aycabta

.travis.yml

@@ -4,9 +4,9 @@ before_install:
   - gem update bundler
 language: ruby
 rvm:
-  - 2.3.7
-  - 2.4.4
-  - 2.5.1
+  - 2.3.8
+  - 2.4.5
+  - 2.5.3
   - ruby-head
   - jruby-9.1.17.0
   - jruby-9.2.0.0