Make raven submit synchronously.

The current implementation has its own low priority thread pool.
We're already executing it in a thread ourselves, so just send
them right away.

I think this causes us to not log fatal errors to Sentry sometimes.

Change-Id: Ib4a105711e71be254e73b9aad1b244fc59fc79a4
Reviewed-on: http://gerrit.dmdirc.com/3328
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Build Manager