Nuke 7.6

• Add Swift 5.0 support – Daniel Storm in #217
• Add SwiftPM 5.0 support - Vadim Shpakovski in #219
• Remove Swift 4.0 and Swift 4.1 support
• Remove iOS 9, tvOS 9, watchOS 2.0, macOS 10.10 and macOS 10.11 support
• Add a single Nuke target which can build the framework for any platform
• Replace deprecated OSAtomic functions with unfair_lock, there are no performance regressions