Fix Bundler installation code

Our previous Bundler idempotency code did not work
if you had a version of Bundler installed on a different Ruby version.
Upgrade newrelic gem

* Specified version gives alert that newrelic "requires a security update for
  their Postgres explain plan support"
* Gem version was last set in July 2013: https://github.com/thoughtbot/suspenders/commit/8f133d2bd0f67097495a9c120f724754ef7b0285#diff-2b85b2e387999d51ca372ef132583e80
Improve memory of app on Heroku w/ New Relic

* Help avoid R14 (Memory quota exceeded) errors.
  http://forum.upcase.com/t/how-to-free-up-swap-space-heroku/3017
* Refactor away some duplication.
Use Heroku-recommended timeout numbers

Rename ENV variables to be very explicit.
Couples their names to their libraries.
Use Rails 4.1.8
Add bundler-audit

We've used this on a couple projects. Here's what it does:

* Scans your Gemfile for insecure dependencies.
* Uses publishd CVEs to find vulnerabilities.

To get set up with bundler-audit, this commit:

* Adds it to the development dependencies.
* Adds a rake task to invoke the CLI.
* Adds it to your default `rake` run.

https://trello.com/c/PltmBueK/210
v1.19.0
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`
Add `bin/deploy` script

* Help avoid forgetting to run `rake db:migrate`.
* Help avoid forgetting to run `restart` after migrate.
* Allow each app to edit these deploy instructions to fit their
  environments' needs (such as `run rake purge` to clear a cache)
  but maintain the same `./bin/deploy` interface and convention
  that is consistent with our many Middleman and Rails apps.
Remove unused `open` definition
Fix spec testing the wrong file
Simplify bin/deploy

- Use the shebang line for POSIX sh.
- Default the argument to 'staging'
- Remove `set -e`.

The `set -e` functionality is not a 100% guarantee of expected semantics
(this is explored more by David Röthlisberger in [Shell scripts: Perils
of “set -e” for error handling][set-e]). In this case, the fact that the
script is two lines means that we can simply add a `&&` to the end of
the first line to get the desired semantics. Not using `set -e` means
that we have to understand how various commands can fail which,
honestly, is a feature.

[set-e]: http://david.rothlis.net/shell-set-e/
Append Markdown to README.md
Use --short