Fix -[RKPropertyMapping isEqualToMapping:] when sourceKeyPath is nil

Closes https://github.com/RestKit/RestKit/issues/1936
Fix -[RKObjectMapping mappingForSourceKeyPath:] when sourceKeyPath is nil

Closes https://github.com/RestKit/RestKit/issues/1937
Update CI rake task
Switch test automation to xctasks
Update to RakeUp 1.2.0
Drop debugger from server
Switch to Expecta HEAD to fix issue with Objective-C++ files
Drop Brewfile
[Travis] Remove `brew bundle` from travis
Fix typo in lint task execution