Travis: Add configuration

[mikegerber]

This adds an (experimental) Travis configuration.

@cneud You may have to activate Travis vs. GitHub ~ like this:

• Go to Travis-ci.org and Sign up with GitHub.
• Accept the Authorization of Travis CI. You’ll be redirected to GitHub.
• Travis-ci.org Settings
  • Activate for the repo

[cneud]

Thanks! I usually prefer CircleCI as Travis has been lacking some environment updates lately, but I have enabled the webhook now.