various improvements

[danvaida]

@mariussturm no time for me right now to create a different branch for the build-badge to match Graylog's Travis account. Please excuse that :)

Although some of the Travis builds are failing(!), I would still suggest merging this PR as it greatly improves* the current state of the integration with Travis for running the tests.

• improves tests runs in Travis CI by making use of build envs. *builds are now running 5 in a row, separate for each distro & ansible version.
• removes duplicate task by making use of OS-family fact
• makes the README a little bit more readable
• paves the way for further enhancements for the tests (i.e. docker images to be used, ansible versions, Travis build matrixes)

If this PR is merged, I'll go ahead and create an issue to fix the tests that are failing (probably because of some docker+systemd quirk). I consider this issue to be out of the scope of this PR.

[mariussturm]

Dude, seriously, please split your PRs up into smaller chunks! There is stuff for at least 3 PRs in here. One for the Travis changes, one for the README update and one formatting PR. I will merge these massive "various improvements" PRs the last time now.