Redirect STDERR from help to /dev/null

On libreoffice > 4, the -h flag attempts to open a UI to display the
help, in addition to writing it to stdout.

This means that on Linux when ran on a server the output ends up looking like:
 Failed to open display
 LibreOffice 4.1.1.2 410m0(Build:2)

This behavior can be avoided by using the `--headless` flag, but that
flag isn't supported in older version of OpenOffice, causing an error
there.

Since Docsplit fails to parse the version string, it assumes it's
OpenOffice in the extract method and uses the JOD convertor, which will
fail with recent versions of LibreOffice.

I believe this is the root cause of issue #71 and should correct it.