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

Author: heyzooi

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:4.2
+// swift-tools-version:4.0
 import PackageDescription
 
 let package = Package(
@@ -17,5 +17,5 @@ let package = Package(
             exclude: ["objc"]
         ),
     ],
-    swiftLanguageVersions: [.v4, .v4_2]
+    swiftLanguageVersions: [4]
 )

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@@ -0,0 +1,21 @@
+// swift-tools-version:4.2
+import PackageDescription
+
+let package = Package(
+    name: "Nimble",
+    products: [
+        .library(name: "Nimble", targets: ["Nimble"]),
+    ],
+    targets: [
+        .target(
+            name: "Nimble", 
+            dependencies: []
+        ),
+        .testTarget(
+            name: "NimbleTests", 
+            dependencies: ["Nimble"], 
+            exclude: ["objc"]
+        ),
+    ],
+    swiftLanguageVersions: [.v4, .v4_2]
+)