Remove warning when compiled w/assertions blocked

This is just #351 as a PR. Not to steal @rodmaz's thunder or anything, but it'd be great to get this warning cleaned up upstream and eventually into our podfile!

From #351: `When CocoaLumberjack is compiled with assertions blocked (RELEASE), a warning is generated by the system on DDASLLogger.m (line 92) as the variable l is never used.`

Thanks!