`help` method failing due to 1.3.6 help message going to STDERR

Apparently this line prints usage information to STDERR. This is the cause of failure of #24.

https://github.com/metanorma/mn2pdf/blob/9317c5aba0a526d2fe0fd4726fb020be87cdb02f/src/main/java/org/metanorma/fop/mn2pdf.java#L183