Fix animation when the transition is .none (#481)

SkeletonView.podspec

@@ -1,6 +1,6 @@
 Pod::Spec.new do |s|
   s.name         = "SkeletonView"
-  s.version      = "1.28.0"
+  s.version      = "1.29.0"
   s.summary      = "An elegant way to show users that something is happening and also prepare them to which contents he is waiting"
   s.description  = <<-DESC
   Today almost all apps have async processes, as API requests, long runing processes, etc. And while the processes are working, usually developers place a loading view to show users that something is going on.

SkeletonViewCore/Sources/Internal/UIKitExtensions/CALayer+Extensions.swift

@@ -78,7 +78,7 @@ extension CALayer {
         insertSublayer(sublayer.contentLayer, at: index)
         switch transition {
         case .none:
-            completion?()
+            DispatchQueue.main.async { completion?() }
         case .crossDissolve(let duration):
             sublayer.contentLayer.setOpacity(from: 0, to: 1, duration: duration, completion: completion)
         }