make SwiftSample support Xcode8

src/objective-c/examples/SwiftSample/SwiftSample.xcodeproj/project.pbxproj

@@ -275,6 +275,7 @@
 				ONLY_ACTIVE_ARCH = YES;
 				SDKROOT = iphoneos;
 				SWIFT_OPTIMIZATION_LEVEL = "-Onone";
+				SWIFT_VERSION = 2.3;
 				TARGETED_DEVICE_FAMILY = "1,2";
 			};
 			name = Debug;
@@ -312,6 +313,7 @@
 				IPHONEOS_DEPLOYMENT_TARGET = 8.4;
 				MTL_ENABLE_DEBUG_INFO = NO;
 				SDKROOT = iphoneos;
+				SWIFT_VERSION = 2.3;
 				TARGETED_DEVICE_FAMILY = "1,2";
 				VALIDATE_PRODUCT = YES;
 			};
@@ -327,6 +329,7 @@
 				PRODUCT_BUNDLE_IDENTIFIER = "io.grpc.$(PRODUCT_NAME:rfc1034identifier)";
 				PRODUCT_NAME = "$(TARGET_NAME)";
 				SWIFT_OBJC_BRIDGING_HEADER = "";
+				SWIFT_VERSION = 2.3;
 				USER_HEADER_SEARCH_PATHS = "";
 			};
 			name = Debug;
@@ -341,6 +344,7 @@
 				PRODUCT_BUNDLE_IDENTIFIER = "io.grpc.$(PRODUCT_NAME:rfc1034identifier)";
 				PRODUCT_NAME = "$(TARGET_NAME)";
 				SWIFT_OBJC_BRIDGING_HEADER = "";
+				SWIFT_VERSION = 2.3;
 				USER_HEADER_SEARCH_PATHS = "";
 			};
 			name = Release;