.travis.yml in benchmark_driver-0.8.6 vs .travis.yml in benchmark_driver-0.9.0

@@ -3,15 +3,13 @@
   - 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
+before_install: gem install bundler -v 1.15.4
 script:
   - bundle exec rake ruby_examples
   - bundle exec rake yaml_examples