Upgrade logging to be compatible with CocoaLumberjack 2.0.0-beta4

Upgrade logging marcos to work with CocoaLumberjack 2.0.0-beta4. Beta 4
is incompitable with beta3 as a result the podspec is updated to rely on
beta 4. It is pinned to beta4 to avoid unexpected breaking changes.
Merge pull request #36 from cloudant/cocoalumberjack_beta4

Upgrade logging to be compatible with CocoaLumberjack 2.0.0-beta4