Switch to use multi json

We are currently experiencing the error below:

```
-- heroku pipeline:promote -a app-name
 !    Heroku client internal error.
 !    Search for help at: https://help.heroku.com
 !    Or report a bug at: https://github.com/heroku/heroku/issues/new

    Error:       uninitialized constant Heroku::OkJson (NameError)
    Backtrace:   /home/ubuntu/.heroku/plugins/heroku-pipeline/lib/heroku/cisaurus/cisaurus.rb:14:in `block in downstreams'
                 /home/ubuntu/.heroku/plugins/heroku-pipeline/lib/heroku/cisaurus/cisaurus.rb:68:in `call'
                 /home/ubuntu/.heroku/plugins/heroku-pipeline/lib/heroku/cisaurus/cisaurus.rb:68:in `handle_error'
                 /home/ubuntu/.heroku/plugins/heroku-pipeline/lib/heroku/cisaurus/cisaurus.rb:13:in `downstreams'
                 /home/ubuntu/.heroku/plugins/heroku-pipeline/lib/heroku/command/pipeline.rb:85:in `promote'
                 /usr/local/heroku/lib/heroku/command.rb:217:in `run'
                 /usr/local/heroku/lib/heroku/cli.rb:38:in `start'
                 /usr/local/heroku/bin/heroku:74:in `<main>'

    Command:     heroku pipeline:promote -a app-name
    Plugins:     heroku-pipeline
    Version:     heroku-toolbelt/3.14.0 (x86_64-linux) ruby/2.1.2
```

This is to switch out `OkJson` to `MultiJson` instead due to the latest changes within the Heroku toolbelt recently at:
https://github.com/heroku/heroku/commit/6dadf8a967ebf3736c1aed1886ebad03be6ee60a