Use xenial instead of trusty
Use xenial instead of trusty

According to
https://blog.travis-ci.com/2019-04-15-xenial-default-build-environment,
xenial is supposed to be the default linux build environment, using
vms. However, our jobs are still using trusty, even though we don't
specify a distribution.

Explicitly use xenial via dist: xenial.

Also remove the sudo option, which historically made sure you were
running on a vm and not in a container. Container-based builds are
deprecated as of
https://changelog.travis-ci.com/the-container-based-build-environment-is-fully-deprecated-84517.

Signed-off-by: Mark Stemm <1b6cbc2e568a933a16c6fa30b73e50cd507debf7@gmail.com>

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