Add ruby-2.4 to .travis.yml

Author: namusyaka

.travis.yml

@@ -4,9 +4,19 @@ sudo: false
 
 matrix:
   include:
-    - rvm: 2.3.3
+    - rvm: 2.4.0
       script:
         - bundle exec danger
+    - rvm: 2.4.0
+      gemfile: Gemfile
+    - rvm: 2.4.0
+      gemfile: gemfiles/rack_edge.gemfile
+    - rvm: 2.4.0
+      gemfile: gemfiles/rack_1.5.2.gemfile
+    - rvm: 2.4.0
+      gemfile: gemfiles/rails_edge.gemfile
+    - rvm: 2.4.0
+      gemfile: gemfiles/rails_5.gemfile
     - rvm: 2.3.3
       gemfile: Gemfile
     - rvm: 2.3.3