Update MailAppDemoSwift to recommended settings

Author: MortimerGoro

demo/MailAppDemoSwift/MailAppDemoSwift.xcodeproj/project.pbxproj

@@ -364,7 +364,7 @@
 				PRODUCT_NAME = "$(TARGET_NAME)";
 				PROVISIONING_PROFILE = "";
 				SWIFT_OBJC_BRIDGING_HEADER = MailAppDemoSwift/ObjCBridgingHeader.h;
-				SWIFT_VERSION = 3.0;
+				SWIFT_VERSION = 4.0;
 			};
 			name = Debug;
 		};
@@ -379,7 +379,7 @@
 				PRODUCT_NAME = "$(TARGET_NAME)";
 				PROVISIONING_PROFILE = "";
 				SWIFT_OBJC_BRIDGING_HEADER = MailAppDemoSwift/ObjCBridgingHeader.h;
-				SWIFT_VERSION = 3.0;
+				SWIFT_VERSION = 4.0;
 			};
 			name = Release;
 		};

demo/MailAppDemoSwift/MailAppDemoSwift/MailViewController.swift

@@ -80,7 +80,7 @@ class MailViewController: UIViewController, UITableViewDataSource, UITableViewDe
     func mailForIndexPath(_ path: IndexPath) -> MailData {
         return demoData[(path as NSIndexPath).row];
     }
-    
+    @objc
     func refreshCallback() {
         prepareDemoData();
         tableView.reloadData();