Swift: Make SwiftMix compatible with Swift 2

Tests/SwiftMix/SwiftMain.swift

@@ -1,8 +1,10 @@
-@objc class SwiftMainClass {
+import Foundation
+
+@objc class SwiftMainClass : NSObject {
   class func SwiftMain(argc:Int, argv:UnsafePointer<UnsafePointer<CChar>>) -> Int32 {
     dump("argc: \(argc)")
     for (var i = 0; i < argc; ++i) {
-      var argi = String.fromCString(argv[i])
+      let argi = String.fromCString(argv[i])
       dump("arg[\(i)]: \(argi)");
     }
     return 0;