Added spec.swift_version to Podspec

Author: malcommac

HydraAsync.podspec

@@ -15,4 +15,5 @@ Pod::Spec.new do |spec|
   spec.requires_arc = true
   spec.module_name = 'Hydra'
   spec.frameworks  = "Foundation"
+  spec.swift_versions = ['4.0', '4.1', '4.2', '5.0', '5.1']
 end

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:5.1
+// swift-tools-version:4.0
 import PackageDescription
 
 let package = Package(