Why I am getting Could not find multi_json-1.3.1 in any of the sources?

Hi folks I have simple Rails application.I want to deploy it in Heroku.When I run the below command

git push heroku master
 

The below error message is displayed.

 Could not find multi_json-1.3.1 in any of the sources
  !
  ! Failed to install gems via Bundler.
  !
  ! Heroku push rejected, failed to compile Ruby/rails app
 

Here is my Gemfile

 gem 'rails', '3.2.3'
  gem 'pg'
  group :assets do
   gem 'sass-rails',   '~> 3.2.3'
   gem 'coffee-rails', '~> 3.2.1'
   gem 'therubyracer', :platform => :ruby
   gem 'uglifier', '>= 1.0.3'
 end
 gem 'jquery-rails'
 

Any help is appreciated.Thank you.

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