Change JSON time format

The [Heroku API Design Guide](https://github.com/interagent/http-api-design#provide-standard-timestamps) states timestamps should use the ISO8601 format. By default, Rails uses millisecond-level precision. This changes it to second-level precision.

Before:
```json
"created_at": "2014-10-20T20:31:29.542Z"
```
After:

```json
"created_at": "2014-10-20T20:31:29Z"
```

We use this guideline at [Intrepid](http://github.com/IntrepidPursuits) and have found it helpful for our APIs.