Merge pull request #715 from aycabta/use-2.6.3

Use Ruby 2.6.3

Author: aycabta

.travis.yml

@@ -7,7 +7,7 @@ rvm:
   - 2.3.8
   - 2.4.6
   - 2.5.5
-  - 2.6.2
+  - 2.6.3
   - ruby-head
   - jruby-9.1.17.0
   - jruby-9.2.6.0