GitLab Developer here, have you tried GitLab CI https://about.gitlab.com/gitlab-ci/? It's not as feature packed as some of the SaaS providers' offerings, but it does the job, runs on your own server, is fully integrated with GitLab.
Thanks Douwe. If people want a CI Saas they can also use Magnum CI that supports GitLab or ci.gitlab.com. The latter is completely free, even for private projects, as long as you bring your own runner to execute the tests.