.travis.yml in hako-0.23.0 vs .travis.yml in hako-0.24.0

@@ -1,10 +1,12 @@
 language: ruby
 sudo: false
 rvm:
-  - 2.3.1
+  - 2.3.3
+  - 2.4.0
   - ruby-head
 before_install:
+  - gem update --system # https://github.com/rubygems/rubygems/pull/1819
   - gem install bundler
 matrix:
   allow_failures:
     - rvm: ruby-head