Add swiftLanguageVersions to Package.swift

Author: mackoj

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:4.0
+// swift-tools-version:5.1
 // The swift-tools-version declares the minimum version of Swift required to build this package.
 
 import PackageDescription
@@ -20,5 +20,6 @@ let package = Package(
       dependencies: [],
       path: "./Sources/AlgoliaSearch-Client"
     ),
-  ]
+  ],
+  swiftLanguageVersions: [.v4_2, .v5]
 )