Merge pull request #40 from yutailang0119/package/support-version

Support macOS to v10_15

Author: yutailang0119

Package.swift

@@ -5,6 +5,9 @@ import PackageDescription
 
 let package = Package(
     name: "ProgressSpinnerKit",
+    platforms: [
+        .macOS(.v10_15),
+    ],
     products: [
         .library(name: "ProgressSpinnerKit", targets: ["ProgressSpinnerKit"]),
     ],