Update Swift Language version to 4.2 under build settings (#551)

* Update swift language version to 4.2 in build settings

* Update podspec

SnapKit.podspec

@@ -14,5 +14,5 @@ Pod::Spec.new do |s|
 
   s.source_files = 'Source/*.swift'
 
-  s.swift_version = '4.0'
+  s.swift_version = '4.2'
 end

SnapKit.xcodeproj/project.pbxproj

@@ -533,7 +533,7 @@
 				SKIP_INSTALL = YES;
 				SWIFT_ACTIVE_COMPILATION_CONDITIONS = DEBUG;
 				SWIFT_OPTIMIZATION_LEVEL = "-Onone";
-				SWIFT_VERSION = 4.0;
+				SWIFT_VERSION = 4.2;
 				VERSIONING_SYSTEM = "apple-generic";
 			};
 			name = Debug;
@@ -553,7 +553,7 @@
 				PRODUCT_NAME = SnapKit;
 				SKIP_INSTALL = YES;
 				SWIFT_OPTIMIZATION_LEVEL = "-Owholemodule";
-				SWIFT_VERSION = 4.0;
+				SWIFT_VERSION = 4.2;
 				VERSIONING_SYSTEM = "apple-generic";
 			};
 			name = Release;