Only build PRs once

We get two builds for each PR, often with different results.

How can we make it so that we get only one build per PR?

I think the solution is [hidden](https://docs.travis-ci.com/user/web-ui/#Build-pushed-pull-requests) in the (somewhat cryptic) documentation

> Build pushed branches #
If ON, builds will be run on branches that are not explicitly excluded in your .travis.yml.
>
>If OFF builds only run on branches that are explicitly included.

and/or

>Build pushed pull requests #
>If ON, builds will be run new pull requests.

Setting "Build pushed pull requests" to __OFF__ for now to see if this solves it.

Catch 77: How to select _which one of the two builds_ are being executed? Since they have different results, the above has a 50:50 chance to disable the wrong one.