[travis] Try to tell gdb the correct perl binary

.travis.yml

@@ -116,4 +116,4 @@ script:
 
 after_failure:
     - COREFILE=$(find . -maxdepth 1 -name "core*" | head -n 1) # find core file
-    - if [[ -f "$COREFILE" ]]; then gdb -c "$COREFILE" example -ex "thread apply all bt" -ex "set pagination 0" -batch; fi
+    - if [[ -f "$COREFILE" ]]; then gdb -c "$COREFILE" `which perl` -ex "thread apply all bt" -ex "set pagination 0" -batch; fi