Fixed SPM Package file

Author: fpillet

Package.swift

@@ -4,7 +4,8 @@ let package = Package(
     name: "Action",
     targets: [],
     dependencies: [
-        .Package(url: "https://github.com/ReactiveX/RxSwift.git", majorVersion: 4)
-    ]
+        .Package(url: "https://github.com/ReactiveX/RxSwift.git", Version(4, 0, 0, prereleaseIdentifiers: ["alpha.1"]))
+    ],
+    exclude: ["Tests/"]
 )