v4.0: Swift 5 and Swift 5 only!

• With release v4.0, MarqueeLabel is now Swift-only! The Objective-C codebase had fallen behind and lost feature parity, and Xcode is very good at ObjC and Swift interop at this point, so I thought it was time. If you want to still use the Objective-C code, you can find it at the new archive repo here: https://github.com/cbpowell/MarqueeLabel-ObjC
• Thanks to @ben9923 for the Swift 5 update!