bypass gelf library level mapping

[ivan-mjch]

This gelf output plugin maps syslog levels to gelf library levels (which I think are ruby logger levels)
and then the gelf library maps them back to syslog levels. This patch avoids the conversion,
storing them as plain integers defines an identity mapping for the gelf library to use.

This patch should avoid the loss of message level information when it is 0 1 or 5.
It works on our systems.

[emsearcy]

I'd prefer if this could just get fixed upstream as per graylog-labs/gelf-rb#52 so we use their library in clean fashion, rather than have to work around it...

emsearcy/gelf-rb@65dd04e

[ivan-mjch]

I didn't see that.

Personally, I would ask for support for using numeric syslog levels directly,
as mapping it back and forth still seems a bit silly, but yes, that should work.