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