Fix for CommonCrypto inclusion in Swift 4.2

Author: jonblatho

Diff for: Package.swift

@@ -4,9 +4,13 @@ import PackageDescription
 
 
 #if os(macOS) || os(iOS) || os(watchOS) || os(tvOS)
+#if canImport(CommonCrypto)
+let dependencies: [Package.Dependency] = []
+#else
 let dependencies = [
   Package.Dependency.package(url: "https://github.com/kylef-archive/CommonCrypto.git", from: "1.0.0"),
 ]
+#endif
 let excludes = ["HMAC/HMACCryptoSwift.swift"]
 let targetDependencies: [Target.Dependency] = []
 #else