Add ruby 2.4 for travis

.travis.yml

@@ -6,14 +6,14 @@ rvm:
   - 2.1
   - 2.2.5
   - 2.3.0
+  - 2.4.0
   - ruby-head
 matrix:
   allow_failures:
     - rvm: ruby-head
     - rvm: 1.9.3
-    - rvm: 2.0
-    - rvm: 2.1
 before_install:
-  - rvm @global do gem install bundler
+  - gem update --system
+  - gem update bundler
 script:
   - bundle exec rubocop && bundle exec rspec