Update podspec to refer a tag

Author: dmcrodrigues

ReactiveCollections.podspec

@@ -10,7 +10,7 @@ Pod::Spec.new do |s|
   s.osx.deployment_target = "10.9"
   s.watchos.deployment_target = "2.0"
   s.tvos.deployment_target = "9.0"
-  s.source       = { :git => "https://github.com/RACCommunity/ReactiveCollections.git", :branch => "master" }
+  s.source       = { :git => "https://github.com/RACCommunity/ReactiveCollections.git", :tag => "#{s.version}" }
 
   # Directory glob for all Swift files
   s.source_files  = "Sources/*.{swift}"