.travis.yml in unasukecop-0.6.1 vs .travis.yml in unasukecop-0.6.2

@@ -1,12 +1,13 @@
 notifications:
   on_success: never
-dist: trusty
+dist: xenial
 sudo: false
 language: ruby
 rvm:
-  - 2.3.6
-  - 2.4.3
-  - 2.5.0
+  - 2.4.5
+  - 2.5.3
+  - 2.6.0
   - ruby-head
-before_install: gem install bundler
+before_install:
+  - gem install bundler --no-document -v 1.17.3
 script: bundle exec rubocop