add swift versions to podspec

Author: chickdan

DownPickerSwift.podspec

@@ -9,6 +9,7 @@ Pod::Spec.new do |s|
  s.source = { :git => "https://github.com/HomebaseAI/DownPickerSwift.git", :tag => s.version.to_s }
  s.platforms = { :ios => "9.0"}
  s.requires_arc = true
+ s.swift_versions = ['4.2', '5.0']
 
  s.default_subspec = "Core"
  s.subspec "Core" do |ss|