added updates to Package.swift so it can be imported with Xcode 11 (#615)

.gitignore

@@ -6,3 +6,4 @@ Examples/
 Gemfile
 Gemfile.lock
 *.sketch
+.swiftpm

Package.swift

@@ -27,11 +27,19 @@ import PackageDescription
 
 let package = Package(
     name: "SnapKit",
+    platforms: [
+        .iOS(.v10),
+        .macOS(.v10_12),
+        .tvOS(.v10)
+    ],
     products: [
         .library(name: "SnapKit", targets: ["SnapKit"]),
     ],
     targets: [
         .target(name: "SnapKit", path: "Source"),
         .testTarget(name: "SnapKitTests", dependencies: ["SnapKit"]),
+    ],
+    swiftLanguageVersions: [
+        .v5
     ]
 )