Use VCR instead of hitting the IrishRail API during tests

Have added VCR to record initial API responses and then to use them in-place of subsequent responses.
This makes the tests more verbose, by needing to wrap all calls in the VCR block, but it means that the test suite is no longer depended on the actual API responses.

The responses are stored in fixtures/vcr_cassettes and can be updated at any time by removing them and rerunning the tests.