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).