Fix issue developing on Ruby 1.9.2.

Basically ActiveSupport requires Ruby 1.9.3+, and SimpleCov requires ActiveSupport.

Since Bundler doesn't let us nail down the Ruby version more precisely than 1.9/2.0, just do coverage analysis on 2.0+.
Fix deprecation warning.
Override DB path in tests when running `bin/bundle-audit`.

(They're horking in Travis trying to write to `/home/travis/.local`...)
Make this resilient to the existence of an empty dir.