Update Repository.master_branch for API changes.

While working on a project using github3.py, I noticed `repo.master_branch` was
always returning `''`, even after doing a `repo.refresh()`.  This didn't seem
right, so I went digging with pdb and found the attribute to be named
`default_branch` instead.

The GitHub API docs[0] use `master_branch` in all their example responses, but
also mention `default_branch`.  I think they changed the name and forgot to
update the documentation.

Since the tests for this project use a static JSON response, there were no test
failures.  I updated the file to what I have experienced, but you should
double-check me on that.

[0]: http://developer.github.com/v3/repos/