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`