Remove .swift-version, add s.swift_version to podspec

.swift-version

@@ -1 +0,0 @@
-3.0

SwiftyStoreKit.podspec

@@ -8,7 +8,7 @@ Pod::Spec.new do |s|
   s.ios.deployment_target = '8.0'
   s.osx.deployment_target = '10.10'
   s.tvos.deployment_target = '9.0' 
-
+  s.swift_version = '4.2'
   s.source       = { :git => "https://github.com/bizz84/SwiftyStoreKit.git", :tag => s.version }
 
   s.source_files = 'SwiftyStoreKit/*.{swift}'