Update Bitbucket Cloud Mirroring REST API usage to version 2.0

On the 29th April 2019, Bitbucket Cloud is to stop using version 1.0 of its REST API.

Details can be found here: https://developer.atlassian.com/cloud/bitbucket/deprecation-notice-v1-apis/

This affects at least the Bitbucket Cloud Mirroring functionality in SR4BB, as that currently uses, in part, the version 1.0 REST API.

We therefore need to update it to use the corresponding version 2.0 REST API before the 29th April 2019.