Fix specs with Ruby 2.0, clean up use of deprecated RSpec features.

Spec fix for Ruby 2.0 (fixing require paths in spec_helper.rb) comes from upstream at https://github.com/heroku/databasedotcom/commit/f402a0d57d6d3223e2fff5f1ebd96a8842ec804c.

Cleanup of deprecated RSpec features from me, because it was hard to tell if the specs were actually running fine or not with all the deprecation noise.

It certainly worked, but while looking up the syntax I realized it could just be `at_least(:once)`, so I've updated it to that.