fix: increase maxBuffer for xorriso child process

[dpgraham]

I got the following error when trying to build AppImage on my Ubuntu VM. The same error occurred in Travis CI.

Error: Exit code: undefined. stderr maxBuffer exceeded
GNU xorriso 1.4.4 : RockRidge filesystem manipulator, libburnia project.

Increasing the maxBuffer size fixed the problem.

[develar]

Thanks!

[dpgraham]

No problem! Thanks for the all the effort you've put into this project.