Check latest data set version

## Is your feature request related to a problem? Please describe.
Similar to #62, but more specific. A call that just returns the latest data set version. I'm thinking this would be useful in R-Shiny dashboards as part of checks to see if there's a newer version of a data set and so the existing cache of data needs clearing.

## Describe the solution you'd like
Looks like there's a couple of end-points that return this:
https://test.statistics.api.education.gov.uk/docs/reference-v1/endpoints/ListDataSetVersions/
https://test.statistics.api.education.gov.uk/docs/reference-v1/endpoints/GetDataSet/

## Describe alternatives you've considered
Could just leave it to solution for #62 and user selects most recent row.