Update Travis CI environment from Ubuntu 14.04 to 20.04 (#159)
Update Travis CI environment from Ubuntu 14.04 to 20.04 (#159)

Since 14.04 is EOL. Travis' default is now `xenial`, but there's both
`bionic` and `focal` available if we request them specifically. (The
latter not yet being fully documented, but still usable and saves us
having to bump again shortly.)

The `sudo: required` line was removed since it's the default now that
Travis no longer supports its rootless container infrastructure.

See:
https://docs.travis-ci.com/user/reference/linux/

Refs W-7496420.
heroku/stack-images