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