[WIP] Use sensu-json instead of json gem

[amdprophet]

This PR replaces the json gem with the sensu-json gem as per #127. There are some limitations that should be considered before this PR should be considered a candidate for merging:

Advantages:

• The same JSON library as Sensu Core & Sensu Enterprise
• Performance benefits

Disadvantages:

• It wraps the Oj gem (except under JRuby) which has C bindings that must be compiled
• Parsed JSON (via Sensu::JSON.load) only allows keys to be accessed by symbols -- I think this may break a lot of plugins

I'm not currently in favour of having this merged as-is. I think we need to ensure keys can be accessed via both strings & symbols. I don't have much of an opinion regarding Oj & C bindings as many Sensu plugins already have gem dependencies with C bindings.

Any feedback, criticism, or corrections will all be greatly appreciated!

[amdprophet]

After thinking about this a bit more and discussing it with @portertech, I'm going to close this PR. I'll investigate using stdlib json and multi-json as alternatives.