Checkout ruby-advisory-db into spec/fixtures

Was getting 18 failures from running specs on a clean checkout. Now 3.

This isolates the DB for specs from the DB in data/ of your working copy, which should make development easier.

Now getting 3 fails locally (same as Travis).  I think the best approach is adding a `--db-path` option and pointing it to `spec/fixtures/ruby-advisory-db` during integration specs. Sound ok?