added multi_json gem

lib/weixin_authorize.rb

 require "rest-client"
+require "multi_json"
 require "weixin_authorize/version"
 require "weixin_authorize/api/user"
 require "weixin_authorize/api/menu"

weixin_authorize.gemspec

@@ -19,7 +19,7 @@ Gem::Specification.new do |spec|
   spec.require_paths = ["lib"]
 
   spec.add_dependency "rest-client", ">= 1.6.7"
-
+  spec.add_runtime_dependency 'multi_json'  , "~> 1.7.2"
   spec.add_development_dependency "bundler", "~> 1.3"
   spec.add_development_dependency "rake"