chore (ci): remove deprecated sudo: false in .travis.yml

https://blog.travis-ci.com/2018-11-19-required-linux-infrastructure-migration

Author: DirtyF

.travis.yml

@@ -1,17 +1,20 @@
-sudo: false
+
 language: ruby
+cache: bundler
 rvm:
   - 2.6
   - 2.4
 branches:
   only:
     - master
+
 before_install:
 - gem update --system
 - gem install bundler
 install:
 - travis_retry script/bootstrap
 script: script/cibuild
+
 notifications:
   irc:
     on_success: change