Just now I've checked on Ubuntu (on Windows bundle update doesn't working) and PDF generated ok.
But looks like when ruby call java mn2pdf application it doesn't redirect message to console, so there aren't any messages for mn2pdf.
I'm not ruby spec at all, but may be need to add an instruction:
{SDTERR=>STDOUT}
See:
https://stackoverflow.com/questions/29829204/how-to-pass-parameters-to-a-jar-file-and-read-the-console-output-in-ruby
https://stackoverflow.com/questions/10354602/how-to-get-java-output-with-ruby?nah=29826744#29826744