Update GitHub Octocat Mark PNG with new branding

This addresses branding update need in #206 and across all slide decks which use the "GitHub" HydeSlides theme.

![screen shot 2013-09-30 at 9 42 02 am](https://f.cloud.github.com/assets/352082/1238325/69aae614-29e7-11e3-8900-b929bfb00536.png)