I had the same issue. The exactly same files work on bitnami stack installed on my windows machine, but gives the bootstrap error on the bitnami's linux virtual machine lamp. I got it working changing:
to
require(dirname(__FILE__).'/bootstrap.php').
I tried to find what was the difference between the two different bitnami stacks, and the only thing I could see is that the the PHP API server are different (FPM/Fast CGI on the stack with bootstrap error, and Apache 2.0 Handler on the working stack). Also the Virtual Directory Support was Disable on the stack with error and Enabled on the good one.
I wonder if this is not some weird bug, or different behavior of the PHP Api servers regarding relative path.
I also had do disable pagespeed module on the virtual machine because the osticket was halting randomly. Commenting these two line on the /opt/bitnami/apache2/conf/httpd.conf, disables the pagespeed module:
#Include conf/pagespeed.conf
#Include conf/pagespeed_libraries.conf