Modify WordPressKit for Swift 5 support

Expected behavior

We should adapt the framework to support Swift 5.

Actual behavior

At the moment the framework uses Swift 4.2.

Steps to reproduce the behavior

For all targets in the project, SWIFT_VERSION is currently set to 4.2.

WordPressKit Environment

• WordPressKit Version : WordPressKit v1.8.0 & before
• OS : iOS 12.x
• Device : all devices
• Xcode Version : Xcode 10.2 beta

Additional Context

Swift 5 Release Notes for Xcode 10.2 beta