Travis: Initiating build for commit '5ada2b9c136fe88fec66f331d03c6b8233cc53ae' on branch 'master': Fixes #5127 Set Base URL used in the AJAX request to `?` instead of `index.php?`

 It is set to '?' rather than 'index.php?' to increase chances that it works
 including for users who have an automatic 301 redirection from index.php? to ?
 POST values are missing when there is such 301 redirection. So by by-passing
 this 301 redirection, we avoid this issue.