Update travis config to remove deprecated style

.travis.yml

@@ -1,8 +1,9 @@
 cache: bundler
-sudo: false
 language: ruby
+dist: xenial
+os: linux
 
-matrix:
+jobs:
   include:
     - rvm: 2.4.9
       env: "CHECK=rubocop"
@@ -16,13 +17,5 @@ matrix:
     - rvm: jruby-9.2.9.0
       env: "CHECK=test"
 
-    - rvm: 2.4.9
-      env: "CHECK=rubocop"
-
-install:
-  - gem update --system
-  - gem install bundler
-  - bundle --version
-  - bundle install --jobs=3 --retry=3 --path=${BUNDLE_PATH:-vendor/bundle}
 script:
   - "bundle exec rake $CHECK"