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/
Use bin/setup from Travis

In order to test the executable documentation provided by bin/setup, we
should use `bin/setup` from CI.

I transitioned checks for gem presence to use `gem list --installed` and
to omit the `--no-document` flag. This flag should be decided by the
user's `.gemrc`.

Stop redirecting STDERR.

Expand grep flags and add --fixed-strings

The expanded flags tell me what this is doing. `--fixed-strings` gives a
tiny speed boost because we're not using regex here.
v.1.20.0