Update .travis.yml (#209)

to test with the latest Ruby release versions

Author: smaeda-ks

.travis.yml

@@ -1,3 +1,4 @@
+dist: xenial
 language: ruby
 
 before_install:
@@ -16,8 +17,8 @@ matrix:
     - rvm: ruby-head
     - rvm: rbx-2
   include:
-    - rvm: 2.6.3
-    - rvm: 2.5.5
-    - rvm: 2.4.6
+    - rvm: 2.6.4
+    - rvm: 2.5.6
+    - rvm: 2.4.7
     - rvm: ruby-head
     - rvm: rbx-2