replace multi_json dependency with regular stdlib JSON

JSON has been part of the ruby stdlib since 1.9.0 (see
https://github.com/ruby/ruby/commit/af1c4167), and the latest version of
google-api-client requires ruby 2.0 or better so we can assume it's available

bin/generate-api

@@ -6,7 +6,6 @@ require 'thor'
 require 'open-uri'
 require 'google/apis/discovery_v1'
 require 'google/apis/generator'
-require 'multi_json'
 require 'logger'
 
 module Google