Switch from 3.7-dev to 3.7 on xenial.
Switch from 3.7-dev to 3.7 on xenial.

Per official TravisCI communications, this is currently the proper way
to support py3.7. It's added as a separate matrix build so all builds
aren't set to use sudo: true and can run in container environments.

See: https://github.com/travis-ci/travis-ci/issues/9815

ahawker/ulid