.travis.yml in benchmark_driver-0.8.2 vs .travis.yml in benchmark_driver-0.8.3

@@ -1,15 +1,13 @@
 language: ruby
 rvm:
   - 2.1.10
   - 2.2.8
-  - 2.3.5
-  - 2.4.2
-  - ruby-head
-cache: bundler
+  - 2.3.6
+  - 2.4.3
+  - 2.5.0
 branches:
   only:
     - master
-before_install: gem install bundler -v 1.15.4
 script:
   - bundle exec rake ruby_examples
   - bundle exec rake yaml_examples