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* Add support for tunneling a local HTTPS server.
* Return a Promise from localtunnel.

* Update node versions in travis.yml
* Use xenial as the default build environment

Resolves audit warning for https://www.npmjs.com/advisories/534

This issue does not directly affect localtunnel since it does not use
the `o` formatter.

Update yargs dependency to enable the new functionality.

per https://github.com/localtunnel/localtunnel/blob/master/lib/Tunnel.js#L17