Update to swift 4.2.1, test on 16.04 / 18.04

[djones6]

Minor updates for testing:

• Update .swift-version to 4.2.1,
• Add Ubuntu 16.04 and 18.04 testing (under Docker),
• Make the .travis.yml more consistent with other IBM-Swift repos (comments, indentation).

[djones6]

Travis is failing because the e-mail credentials (in the form of EMAIL and PASSWORD travis env vars) are not being passed through to the Docker container. Need to think about how we handle this.