Add an explicit dependency on json 1.7.7

On OS X I get the following message when running the specs (caused by
SimpleCov):

    You are using an old or stdlib version of json gem
    Please upgrade to the recent version by adding this to your Gemfile:

      gem 'json', '~> 1.7.7'
Merge pull request #1 from greenalto/bundler

Add an explicit dependency on json 1.7.7