Specify Swift 4.1 as language version

Author: TheCodedSelf

Example/SwiftCronExample.xcodeproj/project.pbxproj

@@ -526,6 +526,7 @@
 				ONLY_ACTIVE_ARCH = YES;
 				SDKROOT = iphoneos;
 				SWIFT_OPTIMIZATION_LEVEL = "-Onone";
+				SWIFT_VERSION = 4.0;
 			};
 			name = Debug;
 		};
@@ -574,6 +575,7 @@
 				MTL_ENABLE_DEBUG_INFO = NO;
 				SDKROOT = iphoneos;
 				SWIFT_OPTIMIZATION_LEVEL = "-Owholemodule";
+				SWIFT_VERSION = 4.0;
 				VALIDATE_PRODUCT = YES;
 			};
 			name = Release;