bump version to 3.2.1 (do not use 3.2.0)

Author: wolfcon

MJExtension.podspec

@@ -1,6 +1,6 @@
 Pod::Spec.new do |s|
   s.name         = "MJExtension"
-  s.version      = "3.2.0"
+  s.version      = "3.2.1"
   s.ios.deployment_target = '8.0'
   s.osx.deployment_target = '10.8'
   s.summary      = "A fast and convenient conversion between JSON and model"