This commit changes travis-api to always return 404 response if resource
is not available. Previously we were returning image/png with "unknown"
status instead if user used "*/*" Accept header, which was confusing.
We can do it, because we use :transaction strategy with DatabaseCleaner,
which starts transaction before each test and rollbacks after it. That
way data before each test is consistent.
The big advantage of such approach is that tests are fast now - we need
to only load Scenario data once.
One of the drawbacks, on the other hand, is that we need to always load
this data, even if no integration tests need running.
We can try to be smart about it and check if any integration tests are
loaded.
