Update deprecated/default Travis configs (#10880)

[`sudo` no longer has any effect][1] and can be removed. [`os` defaults
to `linux` already][2] but should be explicitly set for safety.

[1]: https://blog.travis-ci.com/2018-11-19-required-linux-infrastructure-migration
[2]: https://docs.travis-ci.com/user/reference/overview/#for-a-particular-travisyml-configuration

.travis.yml

@@ -1,5 +1,5 @@
 language: java
-sudo: false
+os: linux
 dist: xenial
 cache:
   directories: