Performed conversion to Swift 4, with recommended Minimize @objc Infe…

…rence option

Xcode 9.4's conversion to Swift 4 made minimal changes for this lib, really just changing the `SWIFT_VERSION` build setting to `4.0` (it made that change on the target; I moved it to project-level).  No additional `@objc`s or whatnot were required.

Tribool.xcodeproj/project.pbxproj

@@ -95,6 +95,11 @@
 			isa = PBXProject;
 			attributes = {
 				LastUpgradeCheck = 9999;
+				TargetAttributes = {
+					OBJ_14 = {
+						LastSwiftMigration = 0940;
+					};
+				};
 			};
 			buildConfigurationList = OBJ_2 /* Build configuration list for PBXProject "Tribool" */;
 			compatibilityVersion = "Xcode 3.2";
@@ -183,7 +188,7 @@
 				SUPPORTED_PLATFORMS = "macosx iphoneos iphonesimulator appletvos appletvsimulator watchos watchsimulator";
 				SWIFT_ACTIVE_COMPILATION_CONDITIONS = SWIFT_PACKAGE;
 				SWIFT_OPTIMIZATION_LEVEL = "-Onone";
-				SWIFT_VERSION = 3.0;
+				SWIFT_VERSION = 4.0;
 				USE_HEADERMAP = NO;
 			};
 			name = Debug;
@@ -204,7 +209,7 @@
 				SUPPORTED_PLATFORMS = "macosx iphoneos iphonesimulator appletvos appletvsimulator watchos watchsimulator";
 				SWIFT_ACTIVE_COMPILATION_CONDITIONS = SWIFT_PACKAGE;
 				SWIFT_OPTIMIZATION_LEVEL = "-O";
-				SWIFT_VERSION = 3.0;
+				SWIFT_VERSION = 4.0;
 				USE_HEADERMAP = NO;
 			};
 			name = Release;