Clean up .travis.yml

This patch cleans up .travis.yml, adds comments to explain some steps
and adds an additional playbook run for testing idempotency.
Idempotency test in .travis.yml put in text block
Make Travis idempotence test display progress
Make Travis idempotence test non-essential
Explain most common fail state in Travis test
Merge pull request #73 from drybjed/travis

Clean up .travis.yml