Merge pull request #76 from beny/podspec-swift-version

Add swift_version to podspec

KeychainSwift.podspec

@@ -19,4 +19,5 @@ Pod::Spec.new do |s|
   s.osx.deployment_target = "10.10"
   s.watchos.deployment_target = "2.0"
   s.tvos.deployment_target = "9.0"
-end
+  s.swift_version = "4.0"
+end