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: `2014-10-20T20:31:29.542Z`

After: `2014-10-20T20:31:29Z`