fix(robot_cli): support running without xvfb-run

Presentation

Use xvfb-run only when available.

Fixes #20 (closed)

Checklist

• Documented:
  • README.md reflects any job, variable or whichever visible change
  • kicker.json reflects any job, variable or whichever visible change
• Tested & examplified:
  • (url to a project sample successfully proving the merge request fixes the issue) https://gitlab.com/ptassin/robotframework-requests

Signed-off-by: Philippe Tassin philippe.tassin@cs-soprasteria.com