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

@@ -1,16 +1,19 @@
 language: ruby
 cache: bundler
-sudo: false
 rvm:
 - 2.6
 - 2.4
-env:
-  global:
-  - NOKOGIRI_USE_SYSTEM_LIBRARIES=true
+
 before_install:
 - gem update --system
 - gem install bundler
+
 before_script: bundle update
 script: script/cibuild
+
+env:
+  global:
+  - NOKOGIRI_USE_SYSTEM_LIBRARIES=true
+
 notifications:
   email: false