ruby 2.1 json

#777

https://drone.io/github.com/marutosi/redmine/16
<pre>
Bundler::GemNotFound: Could not find json-1.8.1.gem for installation
An error occurred while installing json (1.8.1), and Bundler cannot continue.
Make sure that `gem install json -v '1.8.1'` succeeds before bundling.
</pre>