.gitlab-ci.yml: track stable

This way we don't need to update the it on each new release.
3 jobs for master in 8 minutes and 14 seconds (queued for 9 seconds)
Status Job ID Name Coverage
  Test
passed #414596
quicktests

00:01:36

passed #414597
tests-sid

00:08:14

passed #414598
tests-stable

00:07:15