Add bundler:audit rake task description
Change JSON time format

The [Heroku API Design Guide][guide] says timestamps should be ISO8601.
By default, Rails uses millisecond-level precision.
This changes it to second-level precision.

[guide]: https://github.com/interagent/http-api-design#provide-standard-timestamps

* Before: `2014-10-20T20:31:29.542Z`
* After: `2014-10-20T20:31:29Z`
Use Ruby 2.1.5

https://www.ruby-lang.org/en/news/2014/11/13/ruby-2-1-5-is-released/