.travis.yml in benchmark_driver-0.8.3 vs .travis.yml in benchmark_driver-0.8.4

@@ -3,11 +3,15 @@
   - 2.1.10
   - 2.2.8
   - 2.3.6
   - 2.4.3
   - 2.5.0
+  - ruby-head
+cache: bundler
 branches:
   only:
     - master
+before_install:
+  - gem update --system
 script:
   - bundle exec rake ruby_examples
   - bundle exec rake yaml_examples