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

@@ -1,4 +1,5 @@
 language: ruby
+cache: bundler
 rvm:
   - 2.4
   - 2.6
@@ -27,7 +28,6 @@ notifications:
   email:
     on_success: never
     on_failure: change
-sudo: false
-cache: bundler
+
 services:
   - redis-server