Use newest bundler in travis CI

Author: otoyo

.travis.yml

@@ -8,6 +8,10 @@ rvm:
  - 2.3.0
  - jruby
 
+install:
+ - "gem install bundler"
+ - "bundle install --jobs=3 --retry=3"
+
 script:
  - bundle exec rake
  - bundle exec rubocop