Update Package.swift (#923)

* Update Package.swift

Updating Package.swift to add support for Swift 4.2

* adding support for Swift 4.2 without break support for older versions of Swift 4

Package@swift-4.2.swift

@@ -0,0 +1,30 @@
+// swift-tools-version:4.2
+
+import PackageDescription
+
+let pkg = Package(name: "PromiseKit")
+pkg.products = [
+    .library(name: "PromiseKit", targets: ["PromiseKit"]),
+]
+
+let pmk: Target = .target(name: "PromiseKit")
+pmk.path = "Sources"
+pmk.exclude = [
+    "AnyPromise.swift",
+    "AnyPromise.m",
+    "PMKCallVariadicBlock.m",
+    "dispatch_promise.m",
+    "join.m",
+    "when.m",
+    "NSMethodSignatureForBlock.m",
+    "after.m",
+    "hang.m",
+    "race.m",
+    "Deprecations.swift"
+]
+pkg.swiftLanguageVersions = [.v3, .v4, .v4_2]
+pkg.targets = [
+    pmk,
+    .testTarget(name: "A+", dependencies: ["PromiseKit"]),
+    .testTarget(name: "CorePromise", dependencies: ["PromiseKit"], path: "Tests/CorePromise"),
+]