Swift 4.2 in the checked in Xcode project.

The source will always follow the project's compatibility policy, but
this gets the checked in Xcode project to no offer up the Swift upgrade
when it is used.

Author: thomasvl

SwiftProtobuf.xcodeproj/project.pbxproj

@@ -1303,6 +1303,12 @@
 						CreatedOnToolsVersion = 8.0;
 						ProvisioningStyle = Manual;
 					};
+					"______Target_Protobuf" = {
+						LastSwiftMigration = 1010;
+					};
+					"______Target_ProtobufTestSuite" = {
+						LastSwiftMigration = 1010;
+					};
 				};
 			};
 			buildConfigurationList = "___RootConfs_" /* Build configuration list for PBXProject "SwiftProtobuf" */;
@@ -2282,7 +2288,7 @@
 				SKIP_INSTALL = YES;
 				SWIFT_OPTIMIZATION_LEVEL = "-Owholemodule";
 				SWIFT_TREAT_WARNINGS_AS_ERRORS = YES;
-				SWIFT_VERSION = 4.0;
+				SWIFT_VERSION = 4.2;
 				USE_HEADERMAP = NO;
 				VALIDATE_PRODUCT = YES;
 			};
@@ -2344,7 +2350,7 @@
 				SWIFT_ACTIVE_COMPILATION_CONDITIONS = DEBUG;
 				SWIFT_OPTIMIZATION_LEVEL = "-Onone";
 				SWIFT_TREAT_WARNINGS_AS_ERRORS = YES;
-				SWIFT_VERSION = 4.0;
+				SWIFT_VERSION = 4.2;
 				USE_HEADERMAP = NO;
 			};
 			name = Debug;