update CocoaLumberjack to 2.0.0-beta4

due to this commit. https://github.com/CocoaLumberjack/CocoaLumberjack/commit/a4db88cda3929266551e9775db30b1dc9025bb73

DDLogMessage in 2.0.0-beta4 has changed instance variable name. I think KZBootstrapLogFormatter needs to support it, or specify the exact version in dependency. 

I've updated KZBootstrapLogFormatter.m to support the changes.