Convert tests to specs

There are both tests and specs in the application. In disucussion with @tkwidmer and @backspace we settled on preferring to use specs over tests.

To complete:
- [ ] Update README to indicate that new code should be covered with specs.

And convert the following tests to specs:

- [ ] test/controllers/bathrooms_controller_test.rb
- [ ] test/controllers/contacts_controller_test.rb
- [ ] test/controllers/welcome_controller_test.rb
- [ ] test/geocoder_test_helper.rb
- [ ] test/helpers/bathrooms_helper_test.rb
- [ ] test/helpers/contacts_helper_test.rb
- [ ] test/helpers/welcome_helper_test.rb
- [ ] test/models/admin_user_test.rb
- [ ] test/models/bathroom_test.rb
- [ ] test/models/rating_level_test.rb
- [ ] test/test_helper.rb