[Bug] Fix build error

Author: remlostime

Quicksort/Tests/QuicksortTests.swift

@@ -1,6 +1,13 @@
 import XCTest
 
 class QuicksortTests: XCTestCase {
+  func testSwift4() {
+    // last checked with Xcode 9.0b4
+    #if swift(>=4.0)
+      print("Hello, Swift 4!")
+    #endif
+  }
+  
   func testQuicksort() {
     checkSortAlgorithm(quicksort)
   }

Quicksort/Tests/Tests(quicksort).xcodeproj/project.pbxproj

@@ -187,6 +187,7 @@
 				SDKROOT = macosx;
 				SWIFT_OPTIMIZATION_LEVEL = "-Onone";
 				SWIFT_SWIFT3_OBJC_INFERENCE = Off;
+				SWIFT_VERSION = 4.0;
 			};
 			name = Debug;
 		};
@@ -233,6 +234,7 @@
 				SDKROOT = macosx;
 				SWIFT_OPTIMIZATION_LEVEL = "-Owholemodule";
 				SWIFT_SWIFT3_OBJC_INFERENCE = Off;
+				SWIFT_VERSION = 4.0;
 			};
 			name = Release;
 		};