Use latest ruby version in the major versions (#282)

Author: kachick

.travis.yml

@@ -10,11 +10,11 @@ after_success:
 
 matrix:
   include:
-    - rvm: 2.4.1
+    - rvm: 2.4.2
       script:
         - bundle exec danger
-    - rvm: 2.4.1
-    - rvm: 2.3.4
+    - rvm: 2.4.2
+    - rvm: 2.3.5
     - rvm: 2.2.7
     - rvm: ruby-head
     - rvm: jruby-9.1.10.0