Rather than test the commits from the branches the pull request is sent from, we test the merge between the origin and the upstream branch.
The environment variable ${TRAVIS_PULL_REQUEST} is set to "false" when the build is for a normal branch commit. When the build is for a pull request, it will contain the pull request’s number.
Problem
https://github.com/FRosner/spawncamping-dds/pull/256
Fix stated in Travis CI Documentation