Support GitLab CI environment variables

Closes #128 (closed)

I had to update some behaviors on the command as they did not match what the Arkindex Git hook does and would not work in a GitLab CI context:

• The revision author is set to the author's name, not the email;
• One job runs for each branch and for each tag, and on tags the CI_COMMIT_BRANCH is missing, so the revision branch cannot be mandatory;
• The command should have a non-zero exit code when something goes wrong, to cause CI jobs to fail.

I also rewrote the tests so they would not rely on the real CI environment and can run the same both locally and in CI.