Split Travis script into separate jobs and add apt cookbook test

Split Travis script into separate jobs and add apt cookbook test

With kitchen-dokken in place, travis should now successfully run
both the cookstyle script and individual kitchen tests as separate
matrix builds.

Matrix builds will run in parallel, and the global Travis 50min limit
should apply to each separately ensuring that all cookbooks get tested.

https://docs.travis-ci.com/user/build-matrix/