Update Swift PM dependency declaration to Swift 4

Per documentation at https://github.com/apple/swift-package-manager/blob/master/Documentation/PackageDescriptionV4.md#dependencies

I haven't actually used Swift PM, and I'm on iPad so I wasn't able to verify this is correct, but I believe it matches the documentation.

Author: e28eta

README.md

@@ -175,7 +175,7 @@ Installation
 - Add Concurrent to your `Package.swift` file's dependencies section:
 
 ```
-.Package(url: "https://github.com/typelift/Concurrent.git", versions: Version(0,4,0)..<Version(1,0,0))
+.package(url: "https://github.com/typelift/Concurrent.git", "0.4.0"..<"1.0.0")
 ```
 
 #### Carthage