Uses the bundler-audit gem to update the local security database and
show any relevant issues with the app's dependencies. This generator is
only responsible for installing the gem.

The original implementation was written in 2014, and is no longer
relevant. This is because the gem ships with a Rake task that can be
set as the default task, which will be addressed in #1144

Also exposes backup_file and restore_file test helpers into the
public API.