update Ubuntu release for current support version

[mschuchard]

Requirements for Contributing Documentation

• Fill out the template below. Any pull request that does not include enough information to be reviewed in a timely manner may be closed at the maintainers' discretion.
• The pull request must only contribute documentation (for example, markdown files or API docs). To contribute other changes, you must use a different template. You can see all templates at https://github.com/atom/.github/tree/master/.github/PULL_REQUEST_TEMPLATE.

Description of the Change

Updates the example .travis.yml to leverage the currently supported version of Ubuntu for Linux based CI: xenial.

Release Notes

• Update example Travis CI config file to use the currently supported Linux distro.

[rsese]

Thanks for the contribution! We've run into issues supporting older Linux kernels so we intentionally build with Ubuntu 14, see #94 (comment) for more context. Since this is intentional we'll go ahead and close but thanks again for contributing, we do appreciate it.

[mschuchard]

This is Ubuntu 16 and not 18, and works perfectly fine with Atom and its packages due to the actions triggered by #90, but if the update to the documentation for guidance on CI for the package developers is unwanted for some reason, then that is ok too.