Put sync.Mutex on top of variables it protects.
Put sync.Mutex on top of variables it protects.

This change makes the code more consistent with the rateMu below.

This is a followup to https://github.com/google/go-github/pull/390#discussion_r68863652.

Closes #396.

Change-Id: I0fc8b280911f9171fb4d8e3fa60cf6d53b64a65b

google/go-github