Capture Ensembl data version, check REST version

Ensembl REST API can provide current version of REST services, Ensembl data and Ensembl API, see https://github.com/Ensembl/ensembl-rest/wiki/API-Versioning.
We should:
1) save the data version in the DBRefEntry for 'self' for retrieved Ensembl sequences
2) warn the user (once only?) if the REST major version is not the one coded against / expected by Jalview
3) sign up to dev.ensembl.org for early warning of any breaking changes to REST services
4) test in advance against new services to allow a seamless transition when it occurs