Move to Xcode 11 and newer SPM version (#42)

Author: rudro

.travis.yml

@@ -1,5 +1,5 @@
 language: objective-c
-osx_image: xcode10.1
+osx_image: xcode11.2
 
 env:
   - ACTION=test  PLATFORM=Mac     DESTINATION='platform=OS X'

Package.swift

@@ -1,5 +1,4 @@
-// swift-tools-version:4.0
-// The swift-tools-version declares the minimum version of Swift required to build this package.
+// swift-tools-version:5.1
 
 import PackageDescription
 
@@ -22,5 +21,5 @@ let package = Package(
             name: "ConcurrencyTests",
             dependencies: ["Concurrency"]),
     ],
-    swiftLanguageVersions: [4]
+    swiftLanguageVersions: [.v5]
 )