fix: add explicit platform versions to Package.swift (#243)

Package.swift

@@ -23,6 +23,12 @@ import PackageDescription
 
 let package = Package(
   name: "Promises",
+  platforms: [
+    .iOS(.v9),
+    .macOS(.v10_11),
+    .tvOS(.v9),
+    .watchOS(.v2)
+  ],
   products: [
     .library(
       name: "FBLPromises",