Bitbucket turned off their 1.0 API today, which we still used solely for

the fetching of file contents. This was a surprise, but fortunately this

API has since been added to 2.0 (previously, it was 1.0-only).
This change switches us to the 2.0 API for file fetching, and also uses

the faster metadata fetching API for verifying file existence.

Unit tests pass.
Manually tested the new URLs against real repositories, verifying that the

file fetches and validation was working correctly.