some 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.

[CLAassistant]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
8 out of 15 committers have signed the CLA.

:white_check_mark: mariussturm
:white_check_mark: veger
:white_check_mark: danvaida
:white_check_mark: joschi
:white_check_mark: bbaassssiiee
:white_check_mark: srinathman
:white_check_mark: lupupitkanen
:white_check_mark: pablodav
:x: James Carr
:x: jccomputing
:x: strangeman
:x: etcet
:x: anmolnagpal
:x: MartinNowak
:x: rfdrake

---

James Carr, etcet, srinathman seem not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[danvaida]

@mariussturm please delete #69 and #70