When updating comment/PR body, use payload URI.

We were calculating the URL ourselves, which meant chef-repo stuff would
affect the mercury repo, because we were calculating the URL with
respect to the mercury repo. Since the Github API gives us the URL as
part of the payload, we now just use that.
Merge pull request #20 from elemica/repos-take-on-their-multi-armed-form

Repos Take on Their Multi-armed Form: When updating comment/PR body, use payload URI.