Request Bitbucket tags sorted in descending order

[idpaterson]

This change ensures that the latest tag is returned on the first page of the response. It is possible with the default ascending(ish) sort for new tags to appear on the second or later results page causing the plugin to download older tags instead (#752).

For reference, sort=-name should sort correctly for semver tags, not entirely clear how to interpret this for other tag formats:

The name field is handled specially for tags in that, if specified as the sort field, it uses a natural sort order instead of the default lexicographical sort order. For example, it will return ['1.1', '1.2', '1.10'] instead of ['1.1', '1.10', '1.2'].

[afragen]

I have a separate method of adding endpoints to the specific API request but at the moment there aren’t any for Bitbucket.

[idpaterson]

Oops! I'll leave you to filling out the add_endpoints method, that's a bit beyond my level of familiarity with github-updater. At least this pull request works as a quick patch if anyone else is having trouble in the meantime.