Bump version to 2.1.1

Author: onevcat

APNGKit.podspec

@@ -1,7 +1,7 @@
 Pod::Spec.new do |s|
 
   s.name         = "APNGKit"
-  s.version      = "2.1.0"
+  s.version      = "2.1.1"
   s.summary      = "High performance and delightful way to play with APNG format in iOS."
 
   s.description  = <<-DESC

CHANGELOG.md

@@ -4,6 +4,8 @@ All notable changes to this project will be documented in this file.
 
 ## [Unreleased]
 
+## [2.1.1] - 2021-12-15
+
 ### Fix
 
 - An issue introduced in 2.1.0 that the background was not clear before rendering the next frame when `dispose_op` is `previous`. [#118](https://github.com/onevcat/APNGKit/pull/118) @onevcat
@@ -39,3 +41,4 @@ need also check the README to do a re-implement.
 [2.0.1]: https://github.com/onevcat/APNGKit/compare/2.0.0...2.0.1
 [2.0.2]: https://github.com/onevcat/APNGKit/compare/2.0.1...2.0.2
 [2.1.0]: https://github.com/onevcat/APNGKit/compare/2.0.2...2.1.0
+[2.1.1]: https://github.com/onevcat/APNGKit/compare/2.1.0...2.1.1