Convert CI to buildkite

[kekaichinose]

During all but one* Travis run, we see this error:

2.24s$ rake $SUITE
Problem loading gems please install chef and github_changelog_generator
sh -c kitchen test -c 1 chef15-compatible-inspec-ubuntu-1804
-----> Starting Kitchen (v2.3.3)
>>>>>> ------Exception-------
>>>>>> Class: Kitchen::UserError
>>>>>> Message: Vagrant 1.1.0 or higher is not installed. Please download a package from https://www.vagrantup.com/downloads.html.
>>>>>> ----------------------
>>>>>> Please see .kitchen/logs/kitchen.log for more details
>>>>>> Also try running `kitchen diagnose --all` for configuration

The one passing travis run is s a fluke, because it is looking for a failing outcome.

UPDATE: The vagrant error was spurious; it was simply looking at the wrong kitchenfile. #405 gets the tests passing again in Travis.

It remains, however, to convert the CI suite over to Buildkite.