Unique Brewery Names

I added a validator and test for uniqueness of brewery names, in response to [Issue 14](https://github.com/openbeerdatabase/openbeerdatabase/issues/14). When I ran the tests, the uniqueness test I added passed, but a bunch of other tests didn't, throwing this error:

PG::Error: ERROR:  relation "users" does not exist

Not really sure what's going on there, but it seems unrelated to the code in this pull request.