[RFC] coverity: add travis support

Install the coverity addon in Travis-CI. To save resources (coverity scan has an allowance), the static analysis will only be run when pushing to a specific repository.

This is the recommended approach by Coverity itself, but it is not ideal in my opinion. It would be better to just run it once every X days (or once every week) if there has been a new PR. An idea would be to run the coverity addon when pushing to the **master** branch but to let it exit early when it's not "monday" for example. I'm most likely going to need some help with this @jszakmeister and @tarruda. (pinging you guys because you seem to have most experience interacting with travis).

Please do not merge this to master, when this is ready/works, I'll do it myself depending on the route we take with respect to different branches or on master directly.

Also ping @justinmk.

- Documentation: https://scan.coverity.com/travis_ci
- An example project using travis/coverity integration: https://github.com/daksheshvyas/MyHelloWorld
- The script that the coverity addon apparantly executes on travis: https://scan.coverity.com/scripts/travisci_build_coverity_scan.sh