To my surprise Travis CI does ignore the gh-pages branch by default. The branch will not be listed in Travis CI and builds will not be triggered. My problem was that I had a repository with only one gh-pages branch. It contained some static html content that i wanted to html-proof.
Easy solution was to create a master branch instead, change the github pages branch and the github default branch to that master and delete the old gh-pages branch.
Another possible solution would be to explicitly enable the gh-pages branch in the .travis.yml file
branches: gh-pages: true
But i saw no extra beauty in that. Imho it is weird that the gh-pages branch is blacklisted by default.