Automatic tests for docker-compose

As already said in a PR, I want to increase the automatic testing.

So I do want to add Travis CI tests for all of the docker-compose examples. Like we have for the dockerfiles examples.

Therefore I have to download and execute the docker-compose binary on-the-fly during the Travis runs :-)

Start by comparing the existing Travis CI tests for the Dockerfile examples with the docker-compose examples. Identify how the compose binary is downloaded and how each example can be executed during Travis runs. Done means all docker-compose examples run automatically in Travis CI.

Written by the indexing model from the issue text.