[Tests] Drop deprecated `sudo:` option in TravisCI config

That config was deprecated for a while, reference:
- https://blog.travis-ci.com/2018-10-04-combining-linux-infrastructures
diff --git a/.travis.yml b/.travis.yml
index 3deb676..5b83e1b 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,6 +1,5 @@
 language: generic
 dist: xenial
-sudo: required
 addons:
   apt:
     packages: