Fix logo in README
Changing the default branch from `develop` to `master` inadvertently
broke the logo link in the README.
.. image::
- https://raw.github.com/sigmavirus24/github3.py/develop/docs/img/gh3-logo.png
+ https://raw.github.com/sigmavirus24/github3.py/master/docs/img/gh3-logo.png
github3.py is a comprehensive, actively developed and extraordinarily stable
wrapper around the GitHub API (v3).
