Release 2020-01 REST ADMIN API VERSION

[ignacio-chiazzo]

Release 2020-01 REST Admin API.

[jtgrenz]

While perhaps not strictly necessary, it might be a good idea to update https://github.com/Shopify/shopify_api/blob/master/test/fixtures/api_versions.json just for consistency

Alternatively, we could update the gem's test setup to use :define_on_unknown instead and selectively the api version / meta tests to use :raise_on_unknown where necessary.

I can't remember if we set the default test setup this way as an extra check for gem maintainers to verify that the version-specific code they were adding was valid or if it was just an oversight in the transition from the old manually defined versions.