Update .gitlab-ci.yml file and script.

This implements a simpler and more robust policy :

• for merge requests to dev CI is always run
• when the branch name ends in -for-ci, CI is always run. It allows one to trigger the CI from the command line (by pushing to the branch) without having to go through the interface.

Otherwise, the pipeline is not run automatically (but not blocked either, so one can trigger it manually from the web interface).