Rename fork_count repo attribute to forks_count

The API response has forks_count, as can be seen in the API doc at
https://developer.github.com/v3/repos/#response
and the fake data at
https://github.com/sigmavirus24/github3.py/blob/4daa290/tests/unit/test_repos_repo.py#L32
and
https://github.com/sigmavirus24/github3.py/blob/30694d6/tests/json/repo

The previous attribute was kept for backward compatibility, if desired.

Due to the simple nature of the fix and its not being a regression, no test was added, but let me know if one is desired. Perhaps there should be a broader test to ensure module attributes match their corresponding JSON object key.