Travis: fix builds timing out

Since merge of GHSA-mcqj-7p29-9528 (7055731d09ff12b2781410a372f790172e279744),
the builds are timing out during travis_before_script.sh "MantisBT
Installation" step, because curl is not returning any output.

This is due to to introduction of the new path parameter.

Setting it to the actual URL http://$HOSTNAME:$PORT/ does not work, so
we use a blank path.

Fixes 0033791

(cherry picked from commit cd2cf5b3f99b67668bba667e7f918c7a49f04969)