5.2.0 - Swift 5.0

Add

• Compatible with Swift 5.0 and Xcode 10.2. Now Kingfisher builds against Swift 4.0, 4.2 and 5.0. #1098

Fix

• A possible dead lock when using ImagePretcher heavily in another thread. #1122
• Redesign Result type based on Swift Result in standard library. Deprecate value and error getter for Kingfisher.Result.