Add swift_version to podspec

When including into Objective-C only project, there isn't set the SWIFT_VERSION build by default and has to be added manually or in post_install.

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